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.HealthMonitoring.Tests/CentralHealthReportLoopTests.cs

@@ -133,6 +133,83 @@ public class CentralHealthReportLoopTests
         Assert.Equal(fixedInstant.ToUnixTimeMilliseconds(), loop.CurrentSequenceNumber);
     }
 
+    /// <summary>
+    /// HealthMonitoring-018 regression: when <see cref="ICentralHealthAggregator.ProcessReport"/>
+    /// throws, the per-interval counters that
+    /// <see cref="SiteHealthCollector.CollectReport"/> just drained must be
+    /// restored back into the shared collector so they roll forward into the
+    /// next interval rather than being silently lost. Same shape as the
+    /// HealthMonitoring-017 fix in <see cref="HealthReportSender"/>.
+    /// </summary>
+    [Fact]
+    public async Task ProcessReportFailure_PreservesIntervalCountersForNextReport()
+    {
+        var collector = new SiteHealthCollector();
+        // Pre-populate every per-interval counter so the restore path on each
+        // field is exercised. The loop's first iteration will read-and-reset
+        // these via CollectReport, then ProcessReport will throw, and the
+        // restore must put them back.
+        collector.IncrementScriptError();
+        collector.IncrementScriptError();
+        collector.IncrementAlarmError();
+        collector.IncrementDeadLetter();
+        collector.IncrementDeadLetter();
+        collector.IncrementDeadLetter();
+        collector.IncrementSiteAuditWriteFailures();
+        collector.IncrementAuditRedactionFailure();
+        collector.IncrementAuditRedactionFailure();
+
+        var aggregator = new FailingThenSucceedingAggregator();
+        var clusterNodes = new FakeClusterNodeProvider { SelfIsPrimary = true };
+        var options = Options.Create(new HealthMonitoringOptions
+        {
+            ReportInterval = TimeSpan.FromMilliseconds(50)
+        });
+
+        var loop = new CentralHealthReportLoop(
+            collector, aggregator, clusterNodes, options,
+            NullLogger<CentralHealthReportLoop>.Instance);
+
+        await RunLoopBriefly(loop, 450);
+
+        // First call threw, later succeeded — the first successful report
+        // must carry the previously-failed interval's accumulated counts.
+        Assert.NotEmpty(aggregator.Processed);
+        var firstSuccess = aggregator.Processed[0];
+        Assert.Equal(2, firstSuccess.ScriptErrorCount);
+        Assert.Equal(1, firstSuccess.AlarmEvaluationErrorCount);
+        Assert.Equal(3, firstSuccess.DeadLetterCount);
+        Assert.Equal(1, firstSuccess.SiteAuditWriteFailures);
+        Assert.Equal(2, firstSuccess.AuditRedactionFailure);
+    }
+
+    /// <summary>
+    /// <see cref="ICentralHealthAggregator"/> whose first <c>ProcessReport</c>
+    /// call throws (only the first), then subsequent calls succeed. Used by
+    /// <see cref="ProcessReportFailure_PreservesIntervalCountersForNextReport"/>
+    /// to verify the HealthMonitoring-018 restore-on-failure path.
+    /// </summary>
+    private sealed class FailingThenSucceedingAggregator : ICentralHealthAggregator
+    {
+        private int _callCount;
+        public List<SiteHealthReport> Processed { get; } = [];
+
+        public void ProcessReport(SiteHealthReport report)
+        {
+            var n = Interlocked.Increment(ref _callCount);
+            if (n == 1)
+            {
+                throw new InvalidOperationException("aggregator temporarily unavailable");
+            }
+            Processed.Add(report);
+        }
+
+        public void MarkHeartbeat(string siteId, DateTimeOffset receivedAt) { }
+        public IReadOnlyDictionary<string, SiteHealthState> GetAllSiteStates() =>
+            new Dictionary<string, SiteHealthState>();
+        public SiteHealthState? GetSiteState(string siteId) => null;
+    }
+
     [Fact]
     public async Task SetsActiveNodeFlag_EvenWhenNotPrimary()
     {