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.Tests/Gateway/Sessions/SessionManagerTests.cs

@@ -686,6 +686,11 @@ public sealed class SessionManagerTests
         Assert.Equal(1, failingWorkerClient.KillCount);
         Assert.Equal(1, failingWorkerClient.DisposeCount);
         GatewayMetricsSnapshot snapshot = metrics.GetSnapshot();
+
+        // A close-that-failed accounts as SessionClosed (counter += 1) rather
+        // than SessionRemoved (gauge -= 1, counter unchanged). The session is
+        // being removed from the registry on this path, so it must show up in
+        // the closed count.
         Assert.Equal(1, snapshot.SessionsClosed);
         Assert.False(snapshot.EventsBySession.ContainsKey(firstSession.SessionId));
         Assert.Equal(1, snapshot.OpenSessions);
@@ -743,6 +748,9 @@ public sealed class SessionManagerTests
 
     /// <summary>
     /// Verifies that killing a worker removes the session from the registry without calling shutdown.
+    /// Also pins the <c>reason</c> argument propagating through
+    /// <c>SessionManager.KillWorkerAsync</c> → <c>session.KillWorker(reason)</c>
+    /// → <c>IWorkerClient.Kill(reason)</c>.
     /// </summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
@@ -1419,7 +1427,13 @@ public sealed class SessionManagerTests
         /// <summary>Gets the number of times kill was called on the fake worker client.</summary>
         public int KillCount { get; private set; }
 
-        /// <summary>Gets the last reason argument observed by <see cref="Kill"/>.</summary>
+        /// <summary>
+        /// Gets the last reason argument observed by <see cref="Kill"/>. Pins the
+        /// reason-string propagation through <c>SessionManager.KillWorkerAsync</c>
+        /// → <c>session.KillWorker(reason)</c> → <c>IWorkerClient.Kill(reason)</c>;
+        /// without this, the chain could silently drop or substitute the reason
+        /// argument and existing tests would still pass.
+        /// </summary>
         public string? LastKillReason { get; private set; }
 
         /// <summary>Gets the number of times dispose was called on the fake worker client.</summary>