Auto: focas-f3a — cnc_rdalmhistry alarm-history extension

Adds FocasAlarmProjection with two modes (ActiveOnly default, ActivePlusHistory)
that polls cnc_rdalmhistry on connect + on a configurable cadence (5 min default,
HistoryDepth=100 capped at 250). Emits historic events via IAlarmSource with
SourceTimestampUtc set from the CNC's reported timestamp; dedup keyed on
(OccurrenceTime, AlarmNumber, AlarmType). Ships the ODBALMHIS packed-buffer
decoder + encoder in Wire/FocasAlarmHistoryDecoder.cs and threads
ReadAlarmHistoryAsync through IFocasClient (default no-op so existing transport
variants stay back-compat). FocasDriver now implements IAlarmSource.

13 new unit tests cover: mode switch, dedup, distinct-timestamp emission,
type-as-key behaviour, OccurrenceTime passthrough (not Now), HistoryDepth
clamp/fallback, and decoder round-trip. All 341 FOCAS unit tests still pass.

Docs: docs/drivers/FOCAS.md (new), docs/v2/focas-deployment.md (new),
docs/v2/implementation/focas-wire-protocol.md (new),
docs/v2/implementation/focas-simulator-plan.md (new),
docs/drivers/FOCAS-Test-Fixture.md (alarm-history bullet appended).

Closes #267

docs/v2/implementation/focas-simulator-plan.md

@@ -0,0 +1,102 @@
+# FOCAS simulator (focas-mock) plan
+
+Notes on the focas-mock simulator that the FOCAS driver's integration
+tests will eventually talk to. Today there is no FOCAS integration-test
+project; this doc is the contract the future fixture will be built
+against. Keeping the contract tracked in repo means the wire-protocol
+command ids (and their request/response payloads) don't drift between the
+.NET wire client and a future Python implementation.
+
+## Ground rules
+
+- Append-only command ids. Mirror
+  [`focas-wire-protocol.md`](./focas-wire-protocol.md) verbatim.
+- Per-profile state. The simulator hosts N CNC profiles concurrently
+  (`Series0i`, `Series30i`, `PowerMotion`, ...). Each profile has its own
+  alarm-history ring buffer + its own override map.
+- Admin endpoints under `POST /admin/...` mutate state without going
+  through the wire protocol; integration tests use these to seed canned
+  inputs.
+
+## Protocol surface (current scope)
+
+| Cmd | API | State impact |
+| --- | --- | --- |
+| `0x0001` | `cnc_rdcncstat` | reads cached ODBST per profile |
+| `0x0002` | `cnc_rdparam` | reads parameter map per profile |
+| `0x0003` | `cnc_rdmacro` | reads macro variables per profile |
+| `0x0004` | `cnc_rddiag` | reads diagnostic map per profile |
+| `0x0010` | `pmc_rdpmcrng` | reads PMC byte ranges |
+| `0x0020` | `cnc_modal` | reads cached modal MSTB per profile |
+| ... | ... | ... |
+| **`0x0F1A`** | **`cnc_rdalmhistry`** | **dumps the per-profile alarm-history ring buffer (issue #267, plan PR F3-a)** |
+
+## `cnc_rdalmhistry` mock behaviour
+
+The simulator keeps a per-profile ring buffer of alarm-history entries.
+Default fixture seeds 5 profiles with 10 canned entries each (per the F3-a
+plan).
+
+### Request decode
+
+```
+[int16 LE depth]
+```
+
+### Response encode
+
+Use `FocasAlarmHistoryDecoder.Encode` semantics in reverse: emit the
+count followed by `ALMHIS_data` blocks padded to 4-byte boundaries. The
+.NET-side decoder consumes the same format verbatim, so a Python encoder
+written against the table in
+[`focas-wire-protocol.md`](./focas-wire-protocol.md) interoperates without
+extra glue.
+
+### Admin endpoint — `POST /admin/mock_patch_alarmhistory`
+
+Replaces the alarm-history ring buffer for a profile.
+
+```
+POST /admin/mock_patch_alarmhistory
+{
+  "profile": "Series30i",
+  "entries": [
+    {
+      "occurrenceTime": "2025-04-01T09:30:00Z",
+      "axisNo": 1,
+      "alarmType": 2,
+      "alarmNumber": 100,
+      "message": "Spindle overload"
+    },
+    ...
+  ]
+}
+```
+
+`entries` order is interpreted as ring-buffer order (most-recent first to
+match FANUC's natural surface).
+
+### `FocasSimFixture.SeedAlarmHistoryAsync`
+
+The future test-support helper wraps the admin endpoint:
+
+```csharp
+await fixture.SeedAlarmHistoryAsync(
+    profile: "Series30i",
+    entries: new []
+    {
+        new FocasAlarmHistoryEntry(
+            new DateTimeOffset(2025, 4, 1, 9, 30, 0, TimeSpan.Zero),
+            AxisNo: 1, AlarmType: 2, AlarmNumber: 100, Message: "Spindle overload"),
+    });
+```
+
+Integration test `Series/AlarmHistoryProjectionTests.cs` will assert:
+
+- historic events fire once with the seeded timestamps
+- second poll yields zero new events (dedup honoured end-to-end)
+- active-alarm raise/clear still works alongside the history poll
+
+These tests are blocked on the focas-mock + integration-test project
+landing; the unit-test coverage in `FocasAlarmProjectionTests` already
+exercises every same-process invariant.