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/Dashboard/DashboardBrowseAndAlarmModelTests.cs

@@ -171,6 +171,9 @@ public sealed class DashboardBrowseAndAlarmModelTests
         Assert.True(model.IsDegraded);
         Assert.Contains("bg-warning", model.BadgeCssClass, StringComparison.Ordinal);
         Assert.Equal("x", model.Reason);
+
+        // Pin the amber label text, not just the CSS class — a label swap
+        // would otherwise pass this test.
         Assert.Equal(DashboardAlarmProviderStatus.DegradedLabel, model.Label);
     }