docs(alarms): clarify resolver cancellation contract; mark design doc superseded

C6b: IAlarmWatchListResolver.ResolveAsync doc now notes that while discovery being
unavailable never throws, a triggered cancellation token still propagates.
C7: annotate the original design doc where it drifted from the shipped code — metric
names / unimplemented watch-list gauges, and the proto-type location (gateway proto, not
worker proto).

docs/plans/2026-06-13-alarm-subtag-fallback-design.md

@@ -1,7 +1,10 @@
 # Alarm Subtag-Monitoring Fallback — Design
 
 **Date:** 2026-06-13
-**Status:** Approved (brainstorming), ready for implementation planning
+**Status:** Superseded by implementation (merged to `main`). This is the original
+brainstorming design; a few details below were refined during implementation —
+see the inline **Superseded** notes. The shipped behaviour is documented in
+`docs/AlarmClientDiscovery.md`, the client READMEs, and the contracts.
 **Branch:** `feat/alarm-subtag-fallback`
 
 ## Problem
@@ -162,6 +165,11 @@ reconcile cadence and pushes an updated watch-list when the model changes.
 
 **`mxaccess_worker.proto`:**
 
+> **Superseded:** these additions shipped in `mxaccess_gateway.proto`, not
+> `mxaccess_worker.proto` — the worker imports the gateway proto and the alarm
+> commands/events live there (`AlarmSubtagTarget`,
+> `OnAlarmProviderModeChangedEvent`, the extended subscribe command).
+
 - Extend the alarm-subscribe command with: `AlarmProviderMode forced_mode`
   (`UNSPECIFIED` = auto), `int32 consecutive_failure_threshold`,
   `int32 failback_probe_interval_seconds`, `int32 failback_stable_probes`, and
@@ -240,6 +248,12 @@ to `/hubs/alarms`, (c) update metrics, (d) force a reconcile.
 - `mxgateway_alarm_provider_switch_total{from,to,reason}` (counter)
 - `mxgateway_alarm_fallback_watchlist_size` (gauge)
 
+> **Superseded:** the shipped meter names are `mxgateway.alarms.provider_mode`
+> (gauge) and `mxgateway.alarms.provider_switches{from,to,reason}` (counter,
+> `reason` bounded to `failover`/`failback`/`unknown`). The watch-list-size /
+> watch-list-empty gauges were not implemented; an empty watch-list is surfaced
+> via a warning log and the feed's degraded `ProviderStatus` instead.
+
 ## Configuration
 
 ```jsonc

src/ZB.MOM.WW.MxGateway.Server/Alarms/IAlarmWatchListResolver.cs

@@ -19,8 +19,10 @@ public interface IAlarmWatchListResolver
     /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>
     /// <returns>
     ///     The resolved <see cref="AlarmSubtagTarget"/> watch-list, possibly empty.
-    ///     Discovery being unavailable never throws; the caller decides what to do
+    ///     Discovery being unavailable never throws — it yields an empty (or
-    ///     with an empty list.
+    ///     config-only) list and the caller decides what to do with it. Cancellation
+    ///     is the one exception: a triggered <paramref name="cancellationToken"/>
+    ///     still propagates an <see cref="OperationCanceledException"/>.
     /// </returns>
     Task<IReadOnlyList<AlarmSubtagTarget>> ResolveAsync(
         AlarmsOptions options,