fix(error-handling): close Theme 4 — 18 cancellation / fire-and-forget findings

Async cancellation hygiene, fire-and-forget observability, retry/shutdown
semantics, and audit-row coverage across 9 modules. Highlights:

Cancellation & lifecycle:
- AuditLog-006: SqliteAuditWriter.Dispose hops to thread pool, escaping the
  captured SyncContext that risked sync-over-async deadlock.
- AuditLog-010: SiteAuditTelemetryActor owns a private lifecycle CTS,
  threaded through drain paths instead of CancellationToken.None.
- Comm-019: CentralCommunicationActor adds lifecycle CTS for repo calls.
- Host-019: Migration StartupRetry forwards ApplicationStopping so SIGTERM
  during the bounded-retry window aborts cleanly.

Cursor / retry / counter correctness:
- AuditLog-004: SiteAuditReconciliationActor's cursor now holds at `since`
  when any row's idempotent insert is still being retried (per-EventId
  retry counter, MaxPermanentInsertAttempts=5 escape valve with LogCritical
  abandon). No more silent abandonment of permanently-failing rows.
- ConfigDB-019: Dropped the catch-and-continue on EnsureLookaheadAsync's
  SPLIT loop — by class-doc construction the catch could only mask real
  failures and let the next iteration create permanent partition holes.
- HM-017/018: HealthReportSender + CentralHealthReportLoop snapshot
  per-interval counters before sending, restore via new
  ISiteHealthCollector.AddIntervalCounters on transport failure so counts
  aren't silently lost.

Fire-and-forget / shutdown waits:
- InboundAPI-018: AuditWriteMiddleware observes faulted audit-write tasks
  via OnlyOnFaulted continuation (Warning log; response unchanged).
- SnF-024: StoreAndForwardService.StopAsync awaits in-flight retry sweep
  with a bounded SweepShutdownWaitTimeout (10s).

Leak / refactor:
- Comm-021: SiteStreamGrpcServer.SubscribeInstance wraps Subscribe in its
  own try/catch so a throw doesn't leak the relay actor or _activeStreams
  entry.
- Comm-022: VERIFIED already-closed by Comm-016's dead-code purge.
- CLI-017: BundleCommands' three subcommands delegate to ExecuteCommandAsync
  (auth-failure exit-code contract unified).

Defensive / validation:
- CLI-021: CliConfig.Load wraps file-read/JSON parse so malformed config
  prints a warning and returns defaults instead of crashing the CLI.
- Host-022: ParseLevel emits stderr one-shot warning for unrecognised
  MinimumLevel instead of silently coercing to Information.
- ESG-019: ExternalSystemClient sets HttpClient.Timeout=Infinite so the
  per-call CTS is the sole timeout source (was clipped to 100s by .NET).
- Security-020: New SecurityOptionsValidator (IValidateOptions) rejects
  empty LdapServer/LdapSearchBase with ValidateOnStart.
- DM-019: Lifecycle command timeouts now emit DisableTimedOut/EnableTimedOut/
  DeleteTimedOut audit entries (mirrors DeployFailed pattern).

Plus reconciled stale per-module Open-findings counters that had drifted
from prior sessions.

20+ new regression tests across 11 test projects; build clean; affected
suites all green. README regenerated: 75 open (was 93).

tests/ScadaLink.Communication.Tests/Grpc/SiteStreamGrpcServerTests.cs

@@ -308,6 +308,40 @@ public class SiteStreamGrpcServerTests : TestKit
         await streamTask;
     }
 
+    [Fact]
+    public async Task Comm021_SubscribeThrows_StopsRelayActorAndRemovesActiveStreamEntry()
+    {
+        // Communication-021 regression: SubscribeInstance creates a StreamRelayActor
+        // and registers an _activeStreams entry BEFORE calling _streamSubscriber.Subscribe.
+        // If Subscribe throws (e.g. stale instance, site runtime shutting down) and the
+        // pre-fix code lets the throw escape without the wrapping try, the relay actor
+        // and the activeStreams entry both leak. The fix wraps the Subscribe call so the
+        // catch deterministically stops the actor and removes the entry before re-throw.
+        var subscriber = Substitute.For<ISiteStreamSubscriber>();
+        subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
+            .Returns<string>(_ => throw new InvalidOperationException("instance not found"));
+
+        var server = new SiteStreamGrpcServer(subscriber, _logger);
+        server.SetReady(Sys);
+
+        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
+        var context = CreateMockContext();
+
+        // The InvalidOperationException is expected to propagate (the gRPC stack maps
+        // unhandled throws to Internal); the load-bearing assertion is the cleanup.
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => server.SubscribeInstance(MakeRequest("corr-comm021"), writer, context));
+
+        // _activeStreams entry was inserted before Subscribe was called; the catch
+        // must remove it so a follow-up subscription with the same correlation id is
+        // not blocked, and the relay actor must be stopped so it does not leak.
+        Assert.Equal(0, server.ActiveStreamCount);
+
+        // RemoveSubscriber must NOT have been called (Subscribe never returned a
+        // subscription id) — verifying we hit the catch path, not the finally path.
+        subscriber.DidNotReceive().RemoveSubscriber(Arg.Any<IActorRef>());
+    }
+
     [Fact]
     public void SetReady_AllowsStreamCreation()
     {