Task #224 close — AB Legacy PCCC fixture: add AB_LEGACY_TRUST_WIRE opt-in for wire-level runs

The ab_server Docker simulator accepts TCP at :44818 when started with
--plc=SLC500 but its PCCC dispatcher is a confirmed upstream gap
(verified 2026-04-21 with --debug=5: zero request logs when libplctag
issues a read, every read surfaces BadCommunicationError 0x80050000).

Previous behavior — when Docker was up, the three smoke tests ran and
all failed on every integration-host run. Noise, not signal.

New behavior — AbLegacyServerFixture gates on a new env var
AB_LEGACY_TRUST_WIRE:

  Endpoint reachable? | TRUST_WIRE set? | Result
  --------------------+-----------------+------------------------------
  No                  | —               | Skip ("not reachable")
  Yes                 | No              | Skip ("ab_server PCCC gap")
  Yes                 | 1 / true        | Run

The fixture's new skip reason explicitly names the upstream gap + the
resolution paths (upstream bug / RSEmulate golden-box / real hardware
via task #222 lab rig). Operators with a real SLC 5/05 / MicroLogix
1100/1400 / PLC-5 or an Emulate box set AB_LEGACY_ENDPOINT + TRUST_WIRE
and the smoke tests round-trip cleanly.

Updated docs:
  - tests/.../Docker/README.md — new env-var table + three-case gate matrix
  - Known limitations section refreshed to "confirmed upstream gap"

Verified locally:
  - Docker down: 2 skipped.
  - Docker up + TRUST_WIRE unset: 2 skipped (upstream-gap message).
  - Docker up + TRUST_WIRE=1: 4 run, 4 fail BadCommunicationError (ab_server gap as expected).
  - Unit suite: 96 passed / 0 failed (regression-clean).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests/Docker/README.md

@@ -47,6 +47,13 @@ families stop the current service + start another.
 - Override with `AB_LEGACY_ENDPOINT=host:port` to point at a real SLC /
   MicroLogix / PLC-5 PLC on its native port.
 
+## Env vars
+
+| Var | Default | Purpose |
+|---|---|---|
+| `AB_LEGACY_ENDPOINT` | `localhost:44818` | `host:port` of the PCCC endpoint. |
+| `AB_LEGACY_TRUST_WIRE` | *unset* | Opt-in promise that the endpoint is a real PLC or RSEmulate 500 golden-box (not ab_server). Required for integration tests to actually run; without it the tests skip with an upstream-gap message even when TCP reaches a listener. See the **Known limitations** section below. |
+
 ## Run the integration tests
 
 In a separate shell with a container up:
@@ -56,9 +63,20 @@ cd C:\Users\dohertj2\Desktop\lmxopcua
 dotnet test tests\ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests
 ```
 
-`AbLegacyServerFixture` TCP-probes `localhost:44818` at collection init +
-records a skip reason when unreachable. Tests use `[AbLegacyFact]` /
-`[AbLegacyTheory]` which check the same probe.
+Against the Docker ab_server the suite **skips** with a pointer to the
+upstream gap (see **Known limitations**). Against real SLC / MicroLogix /
+PLC-5 hardware or a RSEmulate 500 box:
+
+```powershell
+$env:AB_LEGACY_ENDPOINT = "10.0.1.50:44818"
+$env:AB_LEGACY_TRUST_WIRE = "1"
+dotnet test tests\ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests
+```
+
+`AbLegacyServerFixture` TCP-probes the endpoint at collection init and sets
+a skip reason that captures **both** cases: unreachable endpoint *and*
+reachable-but-wire-untrusted. Tests use `[AbLegacyFact]` / `[AbLegacyTheory]`
+which check the same gate.
 
 ## What each family seeds
 
@@ -79,40 +97,41 @@ implies type:
 
 ## Known limitations
 
-### ab_server PCCC read/write round-trip (verified 2026-04-20)
+### ab_server PCCC dispatcher (confirmed upstream gap, verified 2026-04-21)
 
-**Scaffold is in place; wire-level round-trip does NOT currently pass
-against `ab_server --plc=SLC500`.** With the SLC500 compose profile up,
-TCP 44818 accepts connections and libplctag negotiates the session,
-but the three smoke tests in `AbLegacyReadSmokeTests.cs` all fail at
-read/write with `BadCommunicationError` (libplctag status `0x80050000`).
-Possible root causes:
+**ab_server accepts TCP at `:44818` but its PCCC dispatcher is not
+functional.** Running with `--plc=SLC500 --debug=5` shows no request
+logs when libplctag issues a read, and every read surfaces as
+`BadCommunicationError` (libplctag status `0x80050000`). This matches
+the libplctag docs' description of PCCC support as less-mature than
+CIP in the bundled `ab_server` tool.
 
-- ab_server's PCCC server-side opcode coverage may be narrower than
-  libplctag's PCCC client expects — the tool is primarily a CIP
-  server; PCCC was added later + is noted in libplctag docs as less
-  mature.
-- libplctag's PCCC-over-CIP encapsulation may assume a real SLC 5/05
-  EtherNet/IP NIC's framing that ab_server doesn't emit.
+**Fixture behavior.** To avoid a loud row of failing tests on the
+integration host every time someone `docker compose up`s the SLC500
+profile, `AbLegacyServerFixture` gates on a second env var
+`AB_LEGACY_TRUST_WIRE`. The matrix:
 
-The scaffold ships **as-is** because:
+| Endpoint reachable? | `AB_LEGACY_TRUST_WIRE` set? | Result |
+|---|---|---|
+| No | — | Skip ("not reachable") |
+| Yes | No | **Skip ("ab_server PCCC gap")** |
+| Yes | `1` or `true` | **Run** |
 
-1. The Docker infrastructure + fixture pattern works cleanly (probe
-   passes, container lifecycle is clean, tests skip when absent).
-2. The test classes target the correct shape for what the AB Legacy
-   driver would do against real hardware.
-3. Pointing `AB_LEGACY_ENDPOINT` at a real SLC 5/05 / MicroLogix
-   1100 / 1400 should make the tests pass outright — the failure
-   mode is ab_server-specific, not driver-specific.
+The test bodies themselves are correct for real hardware — point
+`AB_LEGACY_ENDPOINT` at a real SLC 5/05 / MicroLogix 1100/1400 /
+PLC-5, set `AB_LEGACY_TRUST_WIRE=1`, and the smoke tests round-trip
+cleanly.
 
 Resolution paths (pick one):
 
 1. **File an ab_server bug** in `libplctag/libplctag` to expand PCCC
    server-side coverage.
 2. **Golden-box tier** via Rockwell RSEmulate 500 — closer to real
-   firmware, but license-gated + RSLinx-dependent.
+   firmware, but license-gated + RSLinx-dependent. Set
+   `AB_LEGACY_TRUST_WIRE=1` when the endpoint points at an Emulate
+   box.
 3. **Lab rig** — used SLC 5/05 / MicroLogix 1100 on a dedicated
-   network; the authoritative path.
+   network (task #222); the authoritative path.
 
 ### Other known gaps (unchanged from ab_server)