mbproxy: Wave 1 fixes from 2026-05-14 code review

Resolves the four critical correctness defects + the ShutdownCoordinator
double-stop ordering bug called out in codereviews/2026-05-14/Overview.md.
Tests: 362 pass / 0 fail (baseline 358 + 4 new W1 regression tests).

W1.1 — Context swap on running multiplexer.
  PlcMultiplexer._ctx becomes volatile with a new ReplaceContext() method
  that re-registers the cache stats provider on the (preserved) counters.
  PlcListener exposes its multiplexer; PlcListenerSupervisor.ReplaceContextAsync
  swaps the running mux first, then disposes the old cache. Hot-reload
  tag-list changes and the cache-flush-on-reload contract now actually take
  effect on the next PDU instead of waiting for the next listener fault.

W1.2 — Coalescing factory leak.
  When the InFlightByKey factory soft-fails (allocator saturation or duplicate
  TxId), the cleanup path now TryRemoves the stub and walks every party on it
  (including late attachers) to deliver Modbus exception 0x04. Previously
  only the leader got the exception; late attachers waited forever for a
  response that no backend round-trip would ever fire.

W1.3 — Backend-reader head-of-line block.
  UpstreamPipe gains TrySendResponse for non-blocking enqueue. The per-PLC
  backend reader's fan-out loop uses it instead of awaiting SendResponseAsync,
  so a wedged upstream's full bounded response channel can no longer stall
  the single backend reader and starve every other client on that PLC. New
  responseDropForFullUpstream counter on ProxyCounters / CounterSnapshot
  records the drops.

W1.4 — Stranded outbound frames after cascade.
  TearDownBackendAsync acquires _connectGate and drains any frames left in
  _outboundChannel after the writer task faulted/cancelled, releasing their
  proxy TxIds back to the allocator. Without this, a fresh
  EnsureBackendConnectedAsync racing the cascade would send stranded frames
  with old TxIds onto the new backend socket; the responses would arrive
  with no correlation entry and the upstream peers would hang on the
  watchdog until BackendRequestTimeoutMs.

W1.5 — Delete ShutdownCoordinator (Option B).
  Drain logic moved into ProxyWorker.StopAsync. AdminEndpointHost is no
  longer registered as IHostedService; ProxyWorker drives its lifecycle
  directly so admin starts after listeners are bound and stops AFTER the
  in-flight drain (the design's documented contract). Admin is resolved
  lazily in ExecuteAsync to break the circular DI graph
  (Admin -> StatusSnapshotBuilder -> ProxyWorker). GracefulShutdownTimeoutMs
  is now read fresh from IOptionsMonitor.CurrentValue at stop time, so a
  hot-reloaded value is honoured. Removes ShutdownCoordinator + tests.

New tests:
  PlcMultiplexerTests.ReplaceContext_NewTagMap_VisibleOnNextPdu
  PlcMultiplexerTests.ReplaceContext_NewCache_NextReadGoesToBackend_NotOldCache
  UpstreamPipeTests.TrySendResponse_WhenChannelFull_ReturnsFalse_WithoutBlocking
  UpstreamPipeTests.TrySendResponse_AfterDispose_ReturnsFalse

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

mbproxy/src/Mbproxy/Proxy/Multiplexing/UpstreamPipe.cs

@@ -224,6 +224,26 @@ internal sealed partial class UpstreamPipe : IAsyncDisposable
         }
     }
 
+    /// <summary>
+    /// Phase 12 (W1.3) — non-blocking response enqueue. Returns <c>true</c> when the frame
+    /// was queued for delivery, <c>false</c> when the pipe is dead OR the response channel
+    /// is full. Used by the per-PLC backend reader's fan-out loop so a single wedged
+    /// upstream cannot stall responses to peers sharing the same backend socket — without
+    /// this, a full <c>_responseChannel</c> on one pipe would block the reader task.
+    ///
+    /// <para>A <c>false</c> return indicates the frame is the multiplexer's responsibility
+    /// to drop and (optionally) account for via a counter. The wedged upstream's socket
+    /// will eventually time out and close on its own; its read loop will then dispose the
+    /// pipe and the multiplexer's correlation/coalescing entries will be reaped naturally.</para>
+    /// </summary>
+    public bool TrySendResponse(byte[] frame)
+    {
+        if (!IsAlive)
+            return false;
+
+        return _responseChannel.Writer.TryWrite(frame);
+    }
+
     /// <summary>
     /// Closes the pipe: cancels the read+write loops and shuts down the socket. Idempotent.
     /// </summary>