Document the session-less StreamAlarms feed and alarm config

Update the gateway docs for the central alarm monitor reversal:
Grpc.md replaces QueryActiveAlarms with the session-less StreamAlarms
RPC and notes AcknowledgeAlarm no longer needs a session;
Authorization.md maps StreamAlarmsRequest to events:read;
GatewayConfiguration.md adds the MxGateway:Alarms options block; and
GatewayDashboardDesign.md points the Alarms page at the central
monitor cache instead of a per-session subscription.

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

docs/GatewayConfiguration.md

@@ -61,6 +61,12 @@ paths, timeouts, queue sizes, enum values, or protocol values are invalid.
       "ConnectionString": "Server=localhost;Database=ZB;Integrated Security=True;TrustServerCertificate=True;Encrypt=False;",
       "CommandTimeoutSeconds": 60,
       "DashboardRefreshIntervalSeconds": 30
+    },
+    "Alarms": {
+      "Enabled": false,
+      "SubscriptionExpression": "",
+      "DefaultArea": "",
+      "ReconcileIntervalSeconds": 30
     }
   }
 }
@@ -168,6 +174,20 @@ at startup.
 See [Galaxy Repository Browse](./GalaxyRepository.md) for the RPC surface and
 behavior.
 
+## Alarm Options
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `MxGateway:Alarms:Enabled` | `false` | Gates the gateway's always-on central alarm monitor. When `true`, the gateway opens one gateway-owned worker session dedicated to alarms, caches the active-alarm set, and fans it out to every client through the `StreamAlarms` RPC — no client opens its own session to see alarms. |
+| `MxGateway:Alarms:SubscriptionExpression` | _(empty)_ | AVEVA alarm-subscription expression the monitor subscribes on startup, in canonical `\\<machine>\Galaxy!<area>` form. The literal `Galaxy` provider is correct regardless of the Galaxy database name. When empty and `Enabled` is `true`, the gateway falls back to `\\<MachineName>\Galaxy!<DefaultArea>` if `DefaultArea` is set. |
+| `MxGateway:Alarms:DefaultArea` | _(empty)_ | Area name used to compose a default subscription when `SubscriptionExpression` is empty. If both are empty while `Enabled` is `true`, the monitor faults with a configuration diagnostic. |
+| `MxGateway:Alarms:ReconcileIntervalSeconds` | `30` | How often the monitor reconciles its in-process alarm cache against the worker's authoritative active-alarm snapshot, catching transitions the live poll-and-diff feed missed. Floored at 5 seconds. |
+
+The alarm monitor is independent of client sessions: `AcknowledgeAlarm` and
+`StreamAlarms` are session-less RPCs served by the monitor. See
+[Alarm Client Discovery](./AlarmClientDiscovery.md) for the AVEVA consumer
+surface the monitor's worker session drives.
+
 ## Related Documentation
 
 - [Gateway Process Detailed Design](./GatewayProcessDesign.md)