Phase 6.1 Stream E.3 partial — in-flight counter feeds CurrentBulkheadDepth

Closes the observer half of #162 that was flagged as "persisted as 0 today"
in PR #105. The Admin /hosts column refresh + FleetStatusHub SignalR push
+ red-badge visual still belong to the visual-compliance pass.

Core.Resilience:
- DriverResilienceStatusTracker gains RecordCallStart + RecordCallComplete
  + CurrentInFlight field on the snapshot record. Concurrent-safe via the
  same ConcurrentDictionary.AddOrUpdate pattern as the other recorder methods.
  Clamps to zero on over-decrement so a stray Complete-without-Start can't
  drive the counter negative.
- CapabilityInvoker gains an optional statusTracker ctor parameter. When
  wired, every ExecuteAsync / ExecuteAsync(void) wraps the pipeline call
  in try / finally that records start/complete — so the counter advances
  cleanly whether the call succeeds, cancels, or throws. Null tracker keeps
  the pre-Phase-6.1 Stream E.3 behaviour exactly.

Server.Hosting:
- ResilienceStatusPublisherHostedService persists CurrentInFlight as the
  DriverInstanceResilienceStatus.CurrentBulkheadDepth column (was 0 before
  this PR). One-line fix on both the insert + update branches.

The in-flight counter is a pragmatic proxy for Polly's internal bulkhead
depth — a future PR wiring Polly telemetry would replace it with the real
value. The shape of the column + the publisher + the Admin /hosts query
doesn't change, so the follow-up is invisible to consumers.

Tests (8 new InFlightCounterTests, all pass):
- Start+Complete nets to zero.
- Nested starts sum; Complete decrements.
- Complete-without-Start clamps to zero.
- Different hosts track independently.
- Concurrent starts (500 parallel) don't lose count.
- CapabilityInvoker observed-mid-call depth == 1 during a pending call.
- CapabilityInvoker exception path still decrements (try/finally).
- CapabilityInvoker without tracker doesn't throw.

Full solution dotnet test: 1243 passing (was 1235, +8). Pre-existing
Client.CLI Subscribe flake unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Server/Hosting/ResilienceStatusPublisherHostedService.cs

@@ -108,7 +108,7 @@ public sealed class ResilienceStatusPublisherHostedService : BackgroundService
                         HostName = hostName,
                         LastCircuitBreakerOpenUtc = counters.LastBreakerOpenUtc,
                         ConsecutiveFailures = counters.ConsecutiveFailures,
-                        CurrentBulkheadDepth = 0, // Phase 6.1 Stream A tracker doesn't emit bulkhead depth yet
+                        CurrentBulkheadDepth = counters.CurrentInFlight,
                         LastRecycleUtc = counters.LastRecycleUtc,
                         BaselineFootprintBytes = counters.BaselineFootprintBytes,
                         CurrentFootprintBytes = counters.CurrentFootprintBytes,
@@ -119,6 +119,7 @@ public sealed class ResilienceStatusPublisherHostedService : BackgroundService
                 {
                     existing.LastCircuitBreakerOpenUtc = counters.LastBreakerOpenUtc;
                     existing.ConsecutiveFailures = counters.ConsecutiveFailures;
+                    existing.CurrentBulkheadDepth = counters.CurrentInFlight;
                     existing.LastRecycleUtc = counters.LastRecycleUtc;
                     existing.BaselineFootprintBytes = counters.BaselineFootprintBytes;
                     existing.CurrentFootprintBytes = counters.CurrentFootprintBytes;