mbproxy: resolve remaining items from ReReviewAfterRemediation.md

Closes the latent + minor + test-discipline items left after Wave 4. Updates
the re-review doc with a final resolution table — every actionable finding
now marked Resolved or Accepted with rationale.

NM3 — _supervisorCts leaks on re-Start
  StartAsync now disposes the previous CTS before reassigning. Idempotent:
  a try/catch (ObjectDisposedException) covers the very-first-Start case
  where the field-init CTS is still fresh.

NM4 — W2.15 TCS is single-shot
  _firstAttemptCompleted is no longer readonly; StartAsync re-creates it
  after the W2.16 guard so a re-Started supervisor's
  WaitForInitialBindAttemptAsync doesn't observe the previous run's signal.

Nm6 — _admin GetService<> returns null silently
  ProxyWorker.ExecuteAsync now logs a Warning when admin isn't registered.
  Preserves the loud-failure intent from the original IHostedService
  registration without forcing test hosts to wire admin.

Nm7 — AdminEndpointHost.DisposeAsync no double-dispose guard
  Added a volatile bool _disposed flag with an early-return at the top of
  DisposeAsync. Symmetry with PlcMultiplexer; protects against
  ProxyWorker.StopAsync explicitly disposing then DI disposing the singleton
  again on host shutdown.

T3 — RemoveInheritedAppsettings only fires on Build
  AfterTargets="Build;Publish" + a second Delete against $(PublishDir)
  so a `dotnet publish` against the test csproj doesn't ship the example
  PLCs from the linked install template.

T4 — Stale TryAttachOrCreate_*_ReturnsTrue_* test method names
  Renamed to AttachOrCreate_*_WasNew{True,False} after W3 dropped the bool
  return.

Accepted (with rationale documented in ReReviewAfterRemediation.md):
  Nm2 — CoalescedHit semantic is per-design
  Nm4 — _lastBindError preservation on clean exit is intentional forensics
  Nm5 — EventLogBridge has no injectable logger
  Nm8 — Cosmetic log noise
  T1  — Reflection on private fields documented as maintenance trap

Tests: 387 pass / 0 fail.

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

mbproxy/src/Mbproxy/Proxy/ProxyWorker.cs

@@ -233,6 +233,16 @@ internal sealed partial class ProxyWorker : BackgroundService
                 _logger.LogError(ex, "Admin endpoint failed to start: {Message}", ex.Message);
             }
         }
+        else
+        {
+            // Phase 12 (W4 / Nm6) — surface the absence. The previous IHostedService
+            // registration would have hard-errored in DI if AddMbproxyAdmin() was missing
+            // from Program.cs; the W1.5 lazy lookup returns null silently. A single warning
+            // makes a botched composition observable without blocking startup.
+            _logger.LogWarning(
+                "Admin endpoint not registered (AddMbproxyAdmin() missing from composition). " +
+                "Status page will be unavailable; service continues without it.");
+        }
 
         // ── 6. Keep the worker alive until the host signals stop ─────────────────────
         // Supervisors run their own background loops; ExecuteAsync just waits.