# Inbound bit-index RMW writes Implementation Plan

> **For Claude:** REQUIRED SUB-SKILL: Use superpowers-extended-cc:subagent-driven-development to implement this plan task-by-task.

**Goal:** Close the two `stillpending.md` §2 "bit-within-word write" lines — AbLegacy B/I/O-file bit writes and TwinCAT BOOL-within-word writes — both as parent-word read-modify-write.

**Architecture:** AbLegacy already has the full RMW machinery (`WriteBitInWordAsync` + per-parent `GetRmwLock`); it merely excludes B/I/O at the dispatch guard — we drop the exclusion. TwinCAT has no write-side RMW (it throws); we add a driver-level RMW in `TwinCATDriver.WriteAsync` (read the parent word as `UDInt`→`uint`, flip the bit, write it back, under a new per-parent `SemaphoreSlim`) and remove the client-side throw. Both edits are driver-internal: no Commons/wire/proto/EF change.

**Tech Stack:** C# / .NET 10, libplctag (AbLegacy PCCC), Beckhoff TwinCAT.Ads, xUnit + Shouldly. Design: `docs/plans/2026-06-17-stillpending-bit-rmw-writes-design.md` (committed `cf231a78`).

**Branch:** `feat/stillpending-bit-rmw-writes` off master `67da6d4f` (design committed `cf231a78`).

**Dependency graph:** `{T1 ∥ T2} → T3 → T4 → T5`. T1 (AbLegacy) and T2 (TwinCAT) touch disjoint projects → dispatch their implementers concurrently.

**Hard rules (carried from prior phases):** stage by explicit path, never `git add .`; never stage `sql_login.txt` / `src/Server/.../pki/` / `pending.md` / `current.md` / `docker-dev/docker-compose.yml` / `stillpending.md`; never echo/commit secrets; no force-push; no `--no-verify`; **NO EF migration**; **NO Commons wire/proto contract change**; **NO bUnit**; `dangerouslyDisableSandbox` for all build/test/rig commands.

---

### Task 1: AbLegacy B/I/O-file bit writes (drop the exclusion)

**Classification:** standard
**Estimated implement time:** ~4 min
**Parallelizable with:** Task 2

**Files:**
- Modify: `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs:363-373` (the `WriteBitInWordAsync` dispatch guard + its class-doc comment)
- Test: `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs`

**Context:** `WriteBitInWordAsync` (`:603-649`) already does parent-word read → mask → write-back with a per-parent `GetRmwLock`, and works today for N/L/S/A files. The dispatch guard excludes B/I/O:

```csharp
if (def.DataType == AbLegacyDataType.Bit && parsed?.BitIndex is int bit
    && parsed.FileLetter is not "B" and not "I" and not "O")
```

so B/I/O bit writes fall to `runtime.EncodeValue(def.DataType, bitIndex, …)` → the guard throw in `LibplctagLegacyTagRuntime.EncodeValue` (`:138-144`) → `BadNotSupported`. `AbLegacyAddress.TryParse` already accepts `B3:0/0`, `I:0/0`, `O:1/2` (bit range 0..15, `MaxBitIndexFor`), and `(parsed with { BitIndex = null }).ToLibplctagName()` yields `B3:0` / `I:0` / `O:1`. B/I/O are all 16-bit-`Int`-parent files — the existing non-`L` branch handles them with no new code.

**Step 1: Write the failing tests**

Add to `AbLegacyBitRmwTests.cs` (mirror the existing `Bit_set_reads_parent_word_ORs_bit_writes_back` / `Bit_clear_preserves_other_bits_in_N_file_word` pattern, which asserts the parent-word value via `factory.Tags["<parent>"]`):

```csharp
/// <summary>B-file bit set reads the parent word, ORs the bit, writes it back (was BadNotSupported).</summary>
[Theory]
[InlineData("B3:0/3", "B3:0")]
[InlineData("I:0/3", "I:0")]
[InlineData("O:1/3", "O:1")]
public async Task Bit_set_on_B_I_O_file_RMWs_parent_word(string bitAddr, string parentName)
{
    var factory = new FakeAbLegacyTagFactory
    {
        Customise = p => new FakeAbLegacyTag(p) { Value = (short)0b0001 },
    };
    var drv = new AbLegacyDriver(new AbLegacyDriverOptions
    {
        Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
        Tags = [new AbLegacyTagDefinition("F", "ab://10.0.0.5/1,0", bitAddr, AbLegacyDataType.Bit)],
        Probe = new AbLegacyProbeOptions { Enabled = false },
    }, "drv-1", factory);
    await drv.InitializeAsync("{}", CancellationToken.None);

    var results = await drv.WriteAsync([new WriteRequest("F", true)], CancellationToken.None);

    results.Single().StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
    factory.Tags.ShouldContainKey(parentName);
    Convert.ToInt32(factory.Tags[parentName].Value).ShouldBe(0b1001);
}

/// <summary>B-file bit clear preserves the other bits in the parent word.</summary>
[Fact]
public async Task Bit_clear_preserves_other_bits_in_B_file_word()
{
    var factory = new FakeAbLegacyTagFactory
    {
        Customise = p => new FakeAbLegacyTag(p) { Value = unchecked((short)0xFFFF) },
    };
    var drv = new AbLegacyDriver(new AbLegacyDriverOptions
    {
        Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
        Tags = [new AbLegacyTagDefinition("F", "ab://10.0.0.5/1,0", "B3:0/3", AbLegacyDataType.Bit)],
        Probe = new AbLegacyProbeOptions { Enabled = false },
    }, "drv-1", factory);
    await drv.InitializeAsync("{}", CancellationToken.None);

    await drv.WriteAsync([new WriteRequest("F", false)], CancellationToken.None);

    Convert.ToInt32(factory.Tags["B3:0"].Value).ShouldBe(unchecked((short)0xFFF7));
}

/// <summary>Concurrent bit writes to the same B-file word compose correctly (per-parent lock).</summary>
[Fact]
public async Task Concurrent_bit_writes_to_same_B_file_word_compose_correctly()
{
    var factory = new FakeAbLegacyTagFactory
    {
        Customise = p => new FakeAbLegacyTag(p) { Value = (short)0 },
    };
    var tags = Enumerable.Range(0, 8)
        .Select(b => new AbLegacyTagDefinition($"Bit{b}", "ab://10.0.0.5/1,0", $"B3:0/{b}", AbLegacyDataType.Bit))
        .ToArray();
    var drv = new AbLegacyDriver(new AbLegacyDriverOptions
    {
        Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
        Tags = tags,
        Probe = new AbLegacyProbeOptions { Enabled = false },
    }, "drv-1", factory);
    await drv.InitializeAsync("{}", CancellationToken.None);

    await Task.WhenAll(Enumerable.Range(0, 8).Select(b =>
        drv.WriteAsync([new WriteRequest($"Bit{b}", true)], CancellationToken.None)));

    Convert.ToInt32(factory.Tags["B3:0"].Value).ShouldBe(0xFF);
}
```

(Match the exact constructor/field shapes used by the existing tests in this file — copy from `Bit_set_reads_parent_word_ORs_bit_writes_back`. If `FakeAbLegacyTag.Value` for I/O parents needs a `short` like the N-file tests, keep `(short)`.)

**Step 2: Run the tests to verify they fail**

```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests \
  --filter "FullyQualifiedName~AbLegacyBitRmwTests" -v minimal
```
Expected: the new B/I/O tests FAIL — the writes return `BadNotSupported` (parent word unchanged) because B/I/O are excluded from `WriteBitInWordAsync`. Run with `dangerouslyDisableSandbox: true`.

**Step 3: Drop the B/I/O exclusion + fix the comment**

In `AbLegacyDriver.cs`, replace the dispatch block (`:363-373`):

```csharp
// PCCC bit-within-word writes — RMW against a parallel parent-word runtime (strip the /N
// bit suffix). The per-parent-word lock serialises concurrent bit writers. Applies to every
// bit-addressable PCCC file: N-file (N7:0/3), B-file (B3:0/0), and the I/O image files
// (I:0/0, O:1/2); L-file bits RMW a 32-bit parent, the rest a 16-bit word. T/C/R sub-elements
// don't reach this path because they're not Bit typed. NOTE: an Input-image (I) write is sent
// to the PLC like any other write — the device drives the input image from physical inputs and
// may reject it; we surface that real PCCC status rather than pre-rejecting at the driver.
if (def.DataType == AbLegacyDataType.Bit && parsed?.BitIndex is int bit)
{
    results[i] = new WriteResult(
        await WriteBitInWordAsync(device, parsed, bit, w.Value, cancellationToken).ConfigureAwait(false));
    continue;
}
```

(This removes only the `&& parsed.FileLetter is not "B" and not "I" and not "O"` clause and updates the comment. `WriteBitInWordAsync` is unchanged — its `isLong = FileLetter == "L"` already routes B/I/O to the 16-bit `Int` branch.)

**Step 4: Run the tests to verify they pass**

```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests \
  --filter "FullyQualifiedName~AbLegacyBitRmwTests" -v minimal
```
Expected: PASS (new B/I/O + existing N/L tests). Then run the whole AbLegacy suite to confirm no regression:
```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests -v minimal
```
Expected: all green. `dangerouslyDisableSandbox: true`.

**Step 5: Commit**

```bash
git add src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs \
        tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs
git commit -m "feat(ablegacy): B/I/O-file bit-within-word writes via existing RMW path"
```

---

### Task 2: TwinCAT BOOL-within-word RMW writes (driver-level)

**Classification:** standard
**Estimated implement time:** ~5 min
**Parallelizable with:** Task 1

**Files:**
- Modify: `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs` (add a `_bitRmwLocks` field near the other `ConcurrentDictionary` fields ~`:25-28`; add the RMW branch in `WriteAsync` ~`:292-299`)
- Modify: `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/AdsTwinCATClient.cs:182-206` (remove the BOOL-within-word throw; update the `bitIndex` param doc at `:178`)
- Test: `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATReadWriteTests.cs`

**Context:** The driver delegates a bit write straight to the client (`client.WriteValueAsync(symbolName, def.DataType, parsed?.BitIndex, w.Value, ct)`), and `AdsTwinCATClient.WriteValueAsync` (`:189-191`) throws `NotSupportedException` for `Bool + bitIndex`. The real `AdsTwinCATClient` news up a concrete `AdsClient` (not injectable) → RMW buried there is **not** fake-testable. So do the RMW **at the driver level** through two `ITwinCATClient` calls (the fake faithfully stores/returns `Values[parentPath]`, so the full RMW is exercised offline). This mirrors the existing bit-*read* convention (`AdsTwinCATClient.ReadValueAsync:120-128` reads the parent as `uint`); `TwinCATDataType.UDInt` maps to `typeof(uint)` (`MapToClrType:487`), and `TwinCATSymbolPath` is a record so `parsed with { BitIndex = null }` yields the parent path. The per-parent `SemaphoreSlim` spans read+write so concurrent bit-writers don't lose updates (it cannot guard against the PLC program writing the word mid-RMW — inherent, documented).

**Step 1: Write the failing tests**

Add to `TwinCATReadWriteTests.cs` (use the file's `NewDriver(...)` helper + `factory.Customise = () => new FakeTwinCATClient { Values = { … } }` pattern from `Successful_DInt_read_returns_Good_value`; the fake's `WriteLog` records `(symbol, type, bit, value)` and `Values[path]` holds the written value):

```csharp
// ---- BOOL-within-word RMW writes ----

/// <summary>A BOOL-within-word set reads the parent word, ORs the bit, writes it back as UDInt.</summary>
[Fact]
public async Task Bit_set_RMWs_parent_word_as_UDInt()
{
    var (drv, factory) = NewDriver(
        new TwinCATTagDefinition("Flag", "ads://5.23.91.23.1.1:851", "MAIN.Flags.3", TwinCATDataType.Bool));
    factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.Flags"] = 0b0001u } };
    await drv.InitializeAsync("{}", CancellationToken.None);

    var results = await drv.WriteAsync([new WriteRequest("Flag", true)], CancellationToken.None);

    results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.Good);
    factory.Clients[0].Values["MAIN.Flags"].ShouldBe(0b1001u);
    // parent write went out as UDInt with no bit index
    factory.Clients[0].WriteLog.ShouldContain(e =>
        e.symbol == "MAIN.Flags" && e.type == TwinCATDataType.UDInt && e.bit == null);
}

/// <summary>A BOOL-within-word clear preserves the other bits in the parent word.</summary>
[Fact]
public async Task Bit_clear_preserves_other_bits()
{
    var (drv, factory) = NewDriver(
        new TwinCATTagDefinition("Flag", "ads://5.23.91.23.1.1:851", "MAIN.Flags.3", TwinCATDataType.Bool));
    factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.Flags"] = 0xFFFFu } };
    await drv.InitializeAsync("{}", CancellationToken.None);

    await drv.WriteAsync([new WriteRequest("Flag", false)], CancellationToken.None);

    factory.Clients[0].Values["MAIN.Flags"].ShouldBe(0xFFF7u);
}

/// <summary>RMW works on a DWORD parent (bit 20 set above the 16-bit boundary).</summary>
[Fact]
public async Task Bit_set_on_DWORD_parent_sets_high_bit()
{
    var (drv, factory) = NewDriver(
        new TwinCATTagDefinition("Hi", "ads://5.23.91.23.1.1:851", "GVL.Status.20", TwinCATDataType.Bool));
    factory.Customise = () => new FakeTwinCATClient { Values = { ["GVL.Status"] = 0u } };
    await drv.InitializeAsync("{}", CancellationToken.None);

    await drv.WriteAsync([new WriteRequest("Hi", true)], CancellationToken.None);

    factory.Clients[0].Values["GVL.Status"].ShouldBe(1u << 20);
}

/// <summary>A failed parent read short-circuits the RMW and surfaces the read status.</summary>
[Fact]
public async Task Bit_write_surfaces_parent_read_failure()
{
    var (drv, factory) = NewDriver(
        new TwinCATTagDefinition("Flag", "ads://5.23.91.23.1.1:851", "MAIN.Flags.3", TwinCATDataType.Bool));
    factory.Customise = () => new FakeTwinCATClient
    {
        ReadStatuses = { ["MAIN.Flags"] = TwinCATStatusMapper.BadNodeIdUnknown },
    };
    await drv.InitializeAsync("{}", CancellationToken.None);

    var results = await drv.WriteAsync([new WriteRequest("Flag", true)], CancellationToken.None);

    results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
    factory.Clients[0].WriteLog.ShouldBeEmpty(); // no parent write attempted
}

/// <summary>Concurrent bit writes to the same word compose correctly (per-parent lock).</summary>
[Fact]
public async Task Concurrent_bit_writes_to_same_word_compose_correctly()
{
    var tags = Enumerable.Range(0, 8)
        .Select(b => new TwinCATTagDefinition($"Bit{b}", "ads://5.23.91.23.1.1:851", $"MAIN.Flags.{b}", TwinCATDataType.Bool))
        .ToArray();
    var (drv, factory) = NewDriver(tags);
    factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.Flags"] = 0u } };
    await drv.InitializeAsync("{}", CancellationToken.None);

    await Task.WhenAll(Enumerable.Range(0, 8).Select(b =>
        drv.WriteAsync([new WriteRequest($"Bit{b}", true)], CancellationToken.None)));

    Convert.ToUInt32(factory.Clients[0].Values["MAIN.Flags"]).ShouldBe(0xFFu);
}
```

Also **search for an existing test that asserts the old throw** (a Bool+bitIndex write returning `BadNotSupported` / "task #181"):
```bash
grep -rn "BOOL-within\|task #181\|bitIndex.*BadNotSupported\|181" \
  tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/
```
If one exists, update it to the new RMW behavior (it must no longer expect `BadNotSupported`).

**Step 2: Run the tests to verify they fail**

```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests \
  --filter "FullyQualifiedName~TwinCATReadWriteTests" -v minimal
```
Expected: the new RMW tests FAIL — the write currently throws `NotSupportedException` → the driver's `catch (NotSupportedException)` maps to `BadNotSupported` and never RMWs the parent. `dangerouslyDisableSandbox: true`.

**Step 3a: Add the per-parent lock field + RMW branch in `TwinCATDriver`**

Near the other `ConcurrentDictionary` fields (~`:25-28`), add:

```csharp
// Per-parent-word RMW gate for BOOL-within-word writes: a single-bit write is a
// read-modify-write of the parent word (TwinCAT's symbol table doesn't expose "Word.N"),
// so concurrent bit-writers to the same word must serialise or they lose each other's
// updates. Keyed by device + parent symbol. (Cannot guard against the PLC program writing
// the word between our read and write — inherent to RMW.)
private readonly ConcurrentDictionary<string, SemaphoreSlim> _bitRmwLocks =
    new(StringComparer.OrdinalIgnoreCase);
```

In `WriteAsync`, inside the `try` (`:293`), replace the existing 4 lines (`var symbolName = …; var status = await client.WriteValueAsync(…); results[i] = new WriteResult(status);`) with:

```csharp
var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);

// BOOL-within-word write — read-modify-write of the parent word. Mirrors the bit-read
// (AdsTwinCATClient.ReadValueAsync) which reads the parent as uint: read the parent as
// UDInt (-> uint), flip the bit, write it back, all under a per-parent lock.
if (def.DataType == TwinCATDataType.Bool && parsed?.BitIndex is int bit)
{
    var parentPath = (parsed with { BitIndex = null }).ToAdsSymbolName();
    var gate = _bitRmwLocks.GetOrAdd(
        $"{def.DeviceHostAddress}|{parentPath}", static _ => new SemaphoreSlim(1, 1));
    await gate.WaitAsync(cancellationToken).ConfigureAwait(false);
    try
    {
        var (parentValue, readStatus) = await client.ReadValueAsync(
            parentPath, TwinCATDataType.UDInt, null, null, cancellationToken).ConfigureAwait(false);
        if (readStatus != TwinCATStatusMapper.Good)
        {
            results[i] = new WriteResult(readStatus);
        }
        else
        {
            var word = Convert.ToUInt32(parentValue ?? 0u);
            var updated = Convert.ToBoolean(w.Value) ? word | (1u << bit) : word & ~(1u << bit);
            var writeStatus = await client.WriteValueAsync(
                parentPath, TwinCATDataType.UDInt, null, updated, cancellationToken).ConfigureAwait(false);
            results[i] = new WriteResult(writeStatus);
        }
    }
    finally
    {
        gate.Release();
    }
    continue;
}

var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
var status = await client.WriteValueAsync(
    symbolName, def.DataType, parsed?.BitIndex, w.Value, cancellationToken).ConfigureAwait(false);
results[i] = new WriteResult(status);
```

(Confirm `using System.Threading;` is present for `SemaphoreSlim`; `System.Collections.Concurrent` is already imported for the other dictionaries. `Convert.ToBoolean`/`Convert.ToUInt32` mirror the AbLegacy RMW.)

**Step 3b: Remove the client-side throw**

In `AdsTwinCATClient.cs`, delete the guard at `:189-191`:

```csharp
if (bitIndex is int && type == TwinCATDataType.Bool)
    throw new NotSupportedException(
        "BOOL-within-word writes require read-modify-write; tracked in task #181.");
```

so `WriteValueAsync` proceeds straight to `ConvertForWrite` + `_client.WriteValueAsync`. Update the `bitIndex` param doc (`:178`) from `"Optional bit index for BOOL values (not supported for writes)."` to `"Optional bit index for BOOL values. BOOL-within-word writes are handled upstream by TwinCATDriver.WriteAsync as a parent-word read-modify-write, so a bit index does not reach this method on the write path."`

**Step 4: Run the tests to verify they pass**

```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests \
  --filter "FullyQualifiedName~TwinCATReadWriteTests" -v minimal
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests -v minimal
```
Expected: all green (new RMW + existing suites). `dangerouslyDisableSandbox: true`.

**Step 5: Commit**

```bash
git add src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs \
        src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/AdsTwinCATClient.cs \
        tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATReadWriteTests.cs
git commit -m "feat(twincat): BOOL-within-word writes via driver-level parent-word RMW"
```

---

### Task 3: Docs + plan-record §2 clear

**Classification:** small
**Estimated implement time:** ~3 min
**Parallelizable with:** none (after T1+T2)

**Files:**
- Modify: `docs/drivers/AbLegacy.md` (bit-write section — B/I/O now supported + the input-image caveat)
- Modify: `docs/drivers/TwinCAT.md` (BOOL-within-word RMW write + the shared RMW-vs-PLC-program limitation)
- Modify: `docs/plans/2026-06-17-stillpending-bit-rmw-writes.md.tasks.json` (mark T1–T3 status; do **not** stage `stillpending.md`)

**Step 1: Update `docs/drivers/AbLegacy.md`**

Find the write / bit-addressing section. Document that bit-within-word writes (`B3:0/0`, `I:0/0`, `O:1/2`, plus the existing `N7:0/3`, L-file) are supported via parent-word read-modify-write under a per-parent lock; and add the caveat: *an Input-image (`I`) write is sent to the PLC like any other write — the device drives the input image from physical inputs and may reject it, in which case the driver surfaces the device's real PCCC status rather than pre-rejecting.* Note the shared RMW limitation (the lock serialises the driver's bit-writers but not the PLC program writing the word mid-RMW).

**Step 2: Update `docs/drivers/TwinCAT.md`**

Document that BOOL-within-word writes (e.g. `MAIN.Flags.3`) are now supported as a driver-level parent-word read-modify-write (read the parent as `UDInt`/`uint`, flip the bit, write it back) under a per-parent lock, mirroring the existing bit-read. Note the same RMW-vs-PLC-program limitation.

**Step 3: Clear the §2 lines via the plan record (NOT stillpending.md)**

Update `docs/plans/2026-06-17-stillpending-bit-rmw-writes.md.tasks.json` to reflect both §2 lines closed (AbLegacy B/I/O bit writes; TwinCAT BOOL-within-word #181). Do **not** edit or stage `stillpending.md`.

**Step 4: Commit**

```bash
git add docs/drivers/AbLegacy.md docs/drivers/TwinCAT.md \
        docs/plans/2026-06-17-stillpending-bit-rmw-writes.md.tasks.json
git commit -m "docs(drivers): B/I/O + BOOL-within-word bit writes (RMW)"
```

---

### Task 4: Full build + AbLegacy + TwinCAT tests + final integration review

**Classification:** standard
**Estimated implement time:** ~4 min
**Parallelizable with:** none (after T3)

**Files:** none (verification only)

**Step 1: Build the solution**

```bash
dotnet build ZB.MOM.WW.OtOpcUa.slnx -v minimal
```
Expected: build succeeds, no new warnings. `dangerouslyDisableSandbox: true`.

**Step 2: Run both driver suites**

```bash
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests -v minimal
dotnet test tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests -v minimal
```
Expected: all green. Record the counts.

**Step 3: Final integration review**

Dispatch a final integration reviewer over `git diff 67da6d4f..HEAD`. Confirm:
- Both edits are driver-internal — no `IDriver` / `WriteRequest` / `WriteResult` / `ITwinCATClient` / `IAbLegacyTagRuntime` *signature* change, no Commons/wire/proto/EF change.
- AbLegacy: the only behavioral change is B/I/O bit writes now RMW (N/L/S/A unchanged); the input-image caveat is documented.
- TwinCAT: the RMW lives at the driver level and is fake-testable; the client throw is gone; no path can still surface the old `BadNotSupported` for a BOOL-within-word write; the per-parent lock spans read+write (no lost-update window inside the driver) and is released on every exit (incl. the read-failure short-circuit).
- No `git add .`; none of the never-stage files staged.

Apply any Critical/Important fixes via a fresh implementer; re-review.

**Step 4: Commit (if review fixes were applied)**

```bash
git add <only the fixed files by path>
git commit -m "fix(bit-rmw): address final integration review"
```

---

### Task 5: Live `/run` acceptance (best-effort) + finish branch

**Classification:** standard
**Estimated implement time:** ~5 min
**Parallelizable with:** none (after T4)

**Files:** none (verification + merge)

**Step 1: Live `/run` — best-effort, fixture-gated**

There is no local PCCC (libplctag SLC/ML) or ADS (TwinCAT, Windows-only runtime) sim on this Mac, so this slice's primary acceptance gate is **unit-proven** (as 4c did for the non-Modbus drivers).
- Best-effort: if a libplctag `ab_server` emulator can host a writable B-file locally (`docs/drivers/AbServer-Test-Fixture.md`), drive an AbLegacy `B3:0/0` bit set+clear round-trip via the AbLegacy CLI / a skip-gated integration test and confirm the parent word composes. If it can't host a writable B-file, record the honest unit-proven gate + the fixture/operator follow-up.
- TwinCAT live stays operator-gated (needs a TwinCAT/ADS target); record as unit-proven.

Do not block the merge on the fixture — unit coverage (T1 + T2) is the gate; live is a bonus.

**Step 2: Finish the branch — REQUIRED SUB-SKILL: superpowers-extended-cc:finishing-a-development-branch**

Verify the full driver suites are green, then (per the standing meta-choice) **merge to master + push**:
```bash
git checkout master
git pull --ff-only
git merge --ff-only feat/stillpending-bit-rmw-writes
git push
git branch -d feat/stillpending-bit-rmw-writes
```
(If `git checkout master` is blocked by uncommitted branch-only `.tasks.json` edits, commit the bookkeeping to the branch first, then ff-merge — as in Phase 4d.)

**Step 3: Update memory**

Update `project_stillpending_backlog.md` (add the bit-RMW-writes paragraph + update REMAINING) and `MEMORY.md` pointer.