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/drivers/FOCAS.md

@@ -0,0 +1,55 @@
+# FOCAS driver
+
+Fanuc CNC driver for the FS 0i / 16i / 18i / 21i / 30i / 31i / 32i / 35i /
+Power Mate i families. Talks to the controller via the licensed
+`Fwlib32.dll` (Tier C, process-isolated per
+[`docs/v2/driver-stability.md`](../v2/driver-stability.md)).
+
+For range-validation and per-series capability surface see
+[`docs/v2/focas-version-matrix.md`](../v2/focas-version-matrix.md).
+
+## Alarm history (`cnc_rdalmhistry`) — issue #267, plan PR F3-a
+
+`FocasAlarmProjection` exposes two modes via `FocasDriverOptions.AlarmProjection`:
+
+| Mode | Behaviour |
+| --- | --- |
+| `ActiveOnly` *(default)* | Subscribe / unsubscribe / acknowledge wire up so capability negotiation works, but no history poll runs. Back-compat with every pre-F3-a deployment. |
+| `ActivePlusHistory` | On subscribe (== "on connect") and on every `HistoryPollInterval` tick, the projection issues `cnc_rdalmhistry` for the most recent `HistoryDepth` entries. Each previously-unseen entry fires an `OnAlarmEvent` with `SourceTimestampUtc` set from the CNC's reported timestamp — OPC UA dashboards see the real occurrence time, not the moment the projection polled. |
+
+### Config knobs
+
+```jsonc
+{
+  "AlarmProjection": {
+    "Mode": "ActivePlusHistory",        // "ActiveOnly" (default) | "ActivePlusHistory"
+    "HistoryPollInterval": "00:05:00",  // default 5 min
+    "HistoryDepth": 100                 // default 100, capped at 250
+  }
+}
+```
+
+### Dedup key
+
+`(OccurrenceTime, AlarmNumber, AlarmType)`. The same triple across two
+polls only emits once. The dedup set is in-memory and **resets on
+reconnect** — first poll after reconnect re-emits everything in the ring
+buffer. OPC UA clients that need exactly-once semantics dedupe client-side
+on the same triple (the timestamp + type + number tuple is stable across
+the boundary).
+
+### `HistoryDepth` cap
+
+Capped at `FocasAlarmProjectionOptions.MaxHistoryDepth = 250` so an
+operator who types `10000` by accident can't blast the wire session with a
+giant request. Typical FANUC ring buffers cap at ~100 entries; the default
+`HistoryDepth = 100` matches the most common ring-buffer size.
+
+### Wire surface
+
+- Wire-protocol command id: `0x0F1A` (see
+  [`docs/v2/implementation/focas-wire-protocol.md`](../v2/implementation/focas-wire-protocol.md)).
+- ODBALMHIS struct decoder: `Wire/FocasAlarmHistoryDecoder.cs`.
+- Tier-C Fwlib32 backend short-circuits the packed-buffer decoder by
+  surfacing the FWLIB struct fields directly into
+  `FocasAlarmHistoryEntry`.