docs: alarms-over-gateway completion banner + AlarmTracking v2 (PR B.5)

Sixteenth PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Closes the documentation sweep
the plan calls for.

- docs/AlarmTracking.md — promoted top-level v2-final architecture
  doc (was a worktree-only draft pre-epic). Covers the three alarm
  sources (Galaxy MxAccess driver-native / Galaxy sub-attribute
  fallback / scripted alarms), how they converge on
  AlarmConditionService, the Acknowledge routing decision in
  DriverNodeManager (driver-native preferred over IWritable
  sub-attribute fallback), the sidecar historian write-back path
  for non-Galaxy producers, and cross-references to the plan +
  v1 archive.
- docs/v1/AlarmTracking.md — banner pointing readers at the v2
  doc; preserved as historical record.
- docs/drivers/Galaxy.md — capability list updated to include
  IAlarmSource (now eight capabilities, restored by B.2). Replaced
  the "IAlarmSource retired in 7.2" sentence with the restoration
  note + cross-link to docs/AlarmTracking.md.
- docs/plans/alarms-over-gateway.md — completion banner at the
  top of the plan, marking 14 of 16 PRs shipped 2026-04-30 and
  noting that A.2 + A.4 + D.1 are the hardware-gated follow-up.

Memory entries updated separately:
- project_alarms_over_gateway_epic.md (new) — epic summary +
  per-PR digest.
- project_galaxy_via_mxgateway.md — added "Alarms restored"
  bullet pointing at the new architecture.
- project_server_history_alarm_subsystems.md — bullet 2 updated
  to describe the new ack-routing decision (B.3) + bullet 3
  added describing the historian write-back path that B.4 + C.1
  + C.2 light up.
- MEMORY.md index — new pointer entry.

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

docs/v1/AlarmTracking.md

@@ -1,4 +1,12 @@
-# Alarm Tracking
+# Alarm Tracking — v1 archive
+
+> **Historical record.** This document describes the v1 / pre-PR-7.2
+> Galaxy alarm path that ran inside `Galaxy.Host`'s STA pump as
+> `GalaxyAlarmTracker`. PR 7.2 retired the in-process Galaxy stack; the
+> alarms-over-gateway epic (B.2 / B.3 / E.7) restored Galaxy's
+> `IAlarmSource` capability against the new gateway-mediated transport.
+> See [docs/AlarmTracking.md](../AlarmTracking.md) for the v2 final
+> architecture — that is the document to read for current behaviour.
 
 Alarm surfacing is an optional driver capability exposed via `IAlarmSource` (`src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IAlarmSource.cs`). Drivers whose backends have an alarm concept implement it — today: Galaxy (MXAccess alarms), FOCAS (CNC alarms), OPC UA Client (A&C events from the upstream server). Modbus / S7 / AB CIP / AB Legacy / TwinCAT do not implement the interface and the feature is simply absent from their subtrees.