docs(src): restore substantive rationale prose the fca978d marker sweep deleted

Targeted re-read of the 203-file fca978d sweep (docs(src): add missing
XML docs and strip tracking-ID comments): a mechanical pre-pass narrowed
1,383 deletions to 68 files / 816 residual prose lines, and a judged
review of every one found 17 collateral deletions across 10 files —
rationale prose deleted alongside resolved markers with no equivalent
surviving anywhere in the tree. Restored (markers stay stripped, per the
sweep's intent):

- SessionManager: the three metrics-accounting invariants (kill-path
  gauge decrement safety, shutdown kill-fallback registry guard vs
  double bookkeeping, SessionClosed-not-SessionRemoved on failed close)
- SessionManagerTests: the matching accounting expectation note and the
  reason-string propagation pins (test summary + FakeWorkerClient.LastKillReason)
- MxAccessGatewayService.AcknowledgeAlarm: the routing remarks (GUID vs
  Provider!Group.Tag vs InvalidRequest; session-less via IGatewayAlarmService)
  — inheritdoc resolves to nothing (proto-generated base is undocumented)
- HubTokenService.Validate: why the hollow-token guard exists
  (non-empty AuthenticationType alone satisfies IsAuthenticated)
- DashboardSessionAdminService: why both broad catches exist (keep raw
  teardown exceptions out of Blazor's error boundary), Close + Kill paths
- WorkerPipeSession.RunAsync: why the factory result throws instead of
  NREing (unambiguous failure; finally-block Dispose can't no-op)
- LmxSubtagAlarmSource: Advise idempotency; Write is always unsecured
  (user id 0), never WriteSecured semantics
- WnWrapAlarmConsumer: the v1-prefix path is what WIN-911-style code uses
- DashboardSnapshotPublisherTests: what the 10ms slack absorbs
  (Task.Delay's coarse Windows timer quantum)
- DashboardBrowseAndAlarmModelTests: why the label text is pinned, not
  just the CSS class

Everything else flagged verified benign: inheritdoc replacements resolve
to equal-or-richer interface docs, or the substance survives relocated.
NonWindows slnx 0W/0E; touched gateway test classes 65/65.

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManager.cs

@@ -231,6 +231,10 @@ public sealed class SessionManager : ISessionManager
             session.MarkFaulted(exception.Message);
             _metrics.Fault(SessionManagerErrorCode.CloseFailed.ToString());
 
+            // The open-session gauge was incremented in OpenSessionAsync; every
+            // session reaching KillWorkerAsync had SessionOpened recorded. If the
+            // kill path throws, decrement the gauge here so mxgateway.sessions.open
+            // does not leak — mirroring the equivalent guard in OpenSessionAsync.
             _metrics.SessionRemoved();
             await RemoveSessionAsync(session).ConfigureAwait(false);
             throw new SessionManagerException(
@@ -393,6 +397,11 @@ public sealed class SessionManager : ISessionManager
                         "Graceful shutdown failed for session {SessionId}; killing worker.",
                         session.SessionId);
 
+                    // Defensive fallback: CloseSessionCoreAsync's inner
+                    // SessionCloseStartedException catch normally removes the session
+                    // and accounts the close. This outer fallback only fires for
+                    // sessions still in the registry — route through KillWorkerAsync
+                    // so the bookkeeping is identical to the dashboard kill path.
                     if (_registry.TryGet(session.SessionId, out GatewaySession? registeredSession)
                         && registeredSession is not null)
                     {
@@ -443,6 +452,11 @@ public sealed class SessionManager : ISessionManager
             session.MarkFaulted(exception.Message);
             if (!wasClosed)
             {
+                // Account the close as a SessionClosed (decrements the open-session
+                // gauge AND increments the sessions.closed counter), not just
+                // SessionRemoved. The session is being removed from the registry
+                // below; treating this as a half-finished close that only
+                // decremented the gauge would under-count the closed counter.
                 _metrics.SessionClosed();
             }