fix: resolve code-review findings (locally verified)

Server-054/055/056, Contracts-020/021/022, Tests-036/038/039,
IntegrationTests-030/031/032 (+033 deferred to live rig),
Client.Dotnet-026/028/029 (+027 won't-fix), Client.Go-030..034,
Client.Python-032..036, Client.Rust-033..038.

Key fix: SessionEventDistributor orphaned a subscriber that registered after
the pump completed but before disposal (Server-056) -> register paths now
complete late registrants under _lifecycleLock; regression test added. The
racy dashboard-mirror gRPC test made deterministic (Tests-039).

Verified green locally: gateway Tests targeted classes (GatewaySession,
SessionEventDistributor, GatewayOptionsValidator, ProtobufContractRoundTrip,
GatewaySessionDashboardMirror) + dotnet/go/python/rust client suites.

docs/GatewayTesting.md

@@ -51,7 +51,7 @@ shutdown request even when a command or event assertion fails. Cleanup failures
 in that `finally` block are logged rather than thrown, so a real assertion
 failure is never masked by a shutdown timeout.
 
-`WorkerLiveMxAccessSmokeTests` additionally covers five MXAccess parity paths the
+`WorkerLiveMxAccessSmokeTests` additionally covers seven MXAccess parity paths the
 fake-worker tests cannot validate:
 
 - a `Write` round-trip against an advised item, asserting both that the reply is
@@ -67,13 +67,21 @@ fake-worker tests cannot validate:
 - a `WriteSecured` round-trip after `AuthenticateUser`, asserting the reply
   carries `MxCommandKind.WriteSecured` and the credential password never
   appears in the diagnostic message (parity for both the secured-write
-  ordering rule and the "do not log secrets" contract), and
+  ordering rule and the "do not log secrets" contract),
 - an abnormal worker exit (the worker process is killed mid-session) where the
   gateway must transition the session to `SessionState.Faulted` with a
   non-empty fault description carrying a known worker-client classification
-  (pipe disconnected / worker faulted / end-of-stream / heartbeat expired).
+  (pipe disconnected / worker faulted / end-of-stream / heartbeat expired),
+- the B8 new COM commands — `AuthenticateUser`, `ArchestrAUserToId`, `Suspend`,
+  and `Activate` — each asserting a real MXAccess reply (not `InvalidRequest`)
+  is returned against an added-but-not-advised item, and
+- the buffered-data path — `AddBufferedItem` and `SetBufferedUpdateInterval` —
+  asserting the commands round-trip and that the worker delivers at least one
+  `OnBufferedDataChange` event (the empty NoData bootstrap) without crashing
+  or dropping frames; live §3.2 multi-sample conversion is noted as a residual
+  when the rig does not drive sample-bearing buffered batches on demand.
 
-All six tests are gated by the same `MXGATEWAY_RUN_LIVE_MXACCESS_TESTS=1`
+All eight tests are gated by the same `MXGATEWAY_RUN_LIVE_MXACCESS_TESTS=1`
 opt-in variable.
 
 Build the worker before running the smoke: