fix(GWC-37): grade the alarm subtag watch-list instead of latching it silently

The subtag fallback's coverage was fixed at SubscribeAlarms time and never
re-examined. AlarmWatchListResolver deliberately swallows a Galaxy Repository
outage and returns whatever configuration contributed, so a discovery failure at
startup left the monitor subscribed with an empty watch-list for the rest of its
life. In Auto mode the alarmmgr primary keeps working, so nothing looks wrong
until the failover that needs the standby finds it covering zero alarms — with a
single LogWarning emitted possibly weeks earlier as the only evidence.

ResolveAsync now returns an AlarmWatchListResolution (targets + whether discovery
was attempted and whether it failed, with the failure message), which is what the
caller needed to tell "discovery produced nothing" from "nothing was configured".
GatewayAlarmMonitor grades that resolution against the effective mode and folds
the verdict into AlarmProviderStatus, so it reaches the feed, the dashboard badge
and every late-joining subscriber:

- zero targets while the mode can need them (Auto/ForceSubtag) -> degraded with
  SubtagWatchListEmpty; discovery enabled but failing adds SubtagDiscoveryFailed
  and the message. ForceAlarmManager builds no subtag consumer and is not graded.
- degraded is now the OR of "subtag is serving" and "the watch-list covers
  nothing", so the Auto case where alarmmgr is healthy and the standby is empty
  is visible. DashboardAlarmProviderStatus renders that combination as the amber
  "Alarm Manager (no subtag standby)" badge rather than the failover label, which
  would have named the wrong live provider.
- on the failover edge (ApplyProviderModeChangeAsync, toMode == Subtag) the
  monitor re-resolves BEFORE publishing the status and reports any difference as
  SubtagWatchListStale with both counts.

The re-resolution is deliberately NOT re-sent. The worker rejects a second
SubscribeAlarms while subscribed (AlarmCommandHandler.Subscribe), and the only
expressible replacement — UnsubscribeAlarms then SubscribeAlarms — rebuilds
FailoverAlarmConsumer, which restarts on the primary that just failed and
discards the warm standby state, so it would flap the feed rather than repair it.
The finding's other alternative (treat empty coverage as a lifecycle failure) was
rejected too: it would crash-loop the monitor every 5 s on a persistent GR outage
and take the working alarmmgr feed down with it. A monitor restart re-resolves and
re-sends, and the status now says so.

Docs: docs/AlarmClientDiscovery.md gains the grading rules and corrects two stale
claims (the resolver method name, and "re-runs discovery on its reconcile cadence
and pushes an updated watch-list", which never happened); gateway.md gains the
standby-coverage visibility paragraph; docs/GatewayConfiguration.md records the
consequence on the UseGalaxyRepository and Mode rows.

Tests: SubtagCoverage_WhenDiscoveryFailsAtSubscribe_ProviderStatusIsDegradedWith-
Reason and SubtagCoverage_WhenDiscoveryRecoversByFailover_ReResolvesAndReportsDrift
drive the fake-worker monitor; two resolver cases pin the new provenance fields.
The pre-existing dashboard case for degraded+ALARMMGR now asserts the new label.

docs/GatewayConfiguration.md

@@ -445,11 +445,11 @@ native wnwrap alarm-manager provider and the subtag-monitoring fallback.
 
 | Option | Default | Description |
 |--------|---------|-------------|
-| `MxGateway:Alarms:Fallback:Mode` | `Auto` | Provider selection mode. `Auto` uses the alarm manager as primary and fails over to subtag monitoring after consecutive COM failures, then fails back automatically. `ForceAlarmManager` disables failover. `ForceSubtag` forces subtag monitoring on from startup. Values are case-insensitive. |
+| `MxGateway:Alarms:Fallback:Mode` | `Auto` | Provider selection mode. `Auto` uses the alarm manager as primary and fails over to subtag monitoring after consecutive COM failures, then fails back automatically. `ForceAlarmManager` disables failover (and is the one mode whose subtag watch-list is never graded, because no subtag consumer is built). `ForceSubtag` forces subtag monitoring on from startup. Values are case-insensitive. |
 | `MxGateway:Alarms:Fallback:ConsecutiveFailureThreshold` | `3` | Number of consecutive wnwrap COM failures (`COMException` or failure HRESULT from `Subscribe` / `GetXmlCurrentAlarms2`) before the monitor switches to subtag mode. Floored at 1. |
 | `MxGateway:Alarms:Fallback:FailbackProbeIntervalSeconds` | `30` | While in subtag mode, how often (in seconds) the monitor probes the wnwrap provider to detect recovery. Floored at 1. |
 | `MxGateway:Alarms:Fallback:FailbackStableProbes` | `3` | Number of consecutive clean wnwrap probes required before the monitor switches back to the alarm manager. Floored at 1. |
-| `MxGateway:Alarms:Fallback:Discovery:UseGalaxyRepository` | `true` | When `true`, the monitor queries the Galaxy Repository SQL database to build the subtag watch-list for the configured area. |
+| `MxGateway:Alarms:Fallback:Discovery:UseGalaxyRepository` | `true` | When `true`, the monitor queries the Galaxy Repository SQL database to build the subtag watch-list for the configured area. A failed query is not fatal — the monitor continues with whatever configuration contributed — but with `Mode` `Auto` or `ForceSubtag` the resulting coverage gap is reported on the alarm feed's `AlarmProviderStatus` (`degraded = true` with a `Subtag watch-list is empty` / `Galaxy Repository alarm discovery failed` reason) and shown on the dashboard as the amber `Alarm Manager (no subtag standby)` badge, instead of only a startup log line. |
 | `MxGateway:Alarms:Fallback:Discovery:Area` | _(empty)_ | Galaxy area to scope the Repository query to. Falls back to `MxGateway:Alarms:DefaultArea` when empty. Ignored when `UseGalaxyRepository` is `false`. This area is **not** used to compose a Repository-derived alarm's canonical `Galaxy!{area}.{reference}`: each discovered alarm uses its object's real Galaxy area (discovered via `gobject.area_gobject_id`), so the reference's group matches what the native alarmmgr emits. `Discovery:Area` / `DefaultArea` is used as the composition area only for explicit `IncludeAttributes` entries, which carry no discovered area. |
 | `MxGateway:Alarms:Fallback:Discovery:IncludeAttributes` | _(empty)_ | Explicit MXAccess attribute paths to add to the subtag watch-list, supplementing (or replacing, when `UseGalaxyRepository` is `false`) the Repository-derived list. |
 | `MxGateway:Alarms:Fallback:Discovery:ExcludeAttributes` | _(empty)_ | Attribute paths to remove from the merged watch-list (case-insensitive). The exclude is applied after the Repository-derived rows and the explicit `IncludeAttributes` entries are combined, so an exclude that matches an explicit include suppresses it too — excludes win. Ignored when `UseGalaxyRepository` is `false`. |