mbproxy: initial commit through Phase 9 (TxId multiplexing)

Adds the mbproxy service end-to-end. Phases 00-08 implement the
production-ready single-listener / 1:1-backend transparent Modbus TCP
proxy with bidirectional BCD rewriting for the ~54-PLC DL205/DL260
fleet. Phase 9 replaces the connection layer with a single backend
socket per PLC plus MBAP TxId rewriting, lifting the H2-ECOM100's
4-concurrent-client cap as an operational ceiling.

Phase 9 additions of note:
- PlcMultiplexer + UpstreamPipe + TxIdAllocator + CorrelationMap
- InFlightRequest with IReadOnlyList<InterestedParty> (load-bearing
  for Phase 10 read coalescing — do not collapse to a single field)
- Per-request watchdog: surfaces Modbus exception 0x0B to upstream
  on BackendRequestTimeoutMs, defending against lost responses,
  dead-PLC paths, and pymodbus 3.13.0's concurrent-multiplexed-
  request bug (its ServerRequestHandler.last_pdu state race)
- Status DTO + HTML gain inFlight / maxInFlight / txIdWraps /
  disconnectCascades / queueDepth (Tier 1.6 in docs/kpi.md)

Tests: 263 unit + 38 E2E. Multiplexer correctness under truly
concurrent backend traffic is proved against a stub backend in
PlcMultiplexerTests; MultiplexerE2ETests paces requests so pymodbus
3.13's single-PDU framer stays in known-good mode.

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

mbproxy/src/Mbproxy/Proxy/Supervision/SupervisorState.cs

@@ -0,0 +1,50 @@
+namespace Mbproxy.Proxy.Supervision;
+
+/// <summary>
+/// State machine states for <see cref="PlcListenerSupervisor"/>.
+/// </summary>
+public enum SupervisorState
+{
+    /// <summary>
+    /// The listener is bound and its accept loop is running.
+    /// Entry conditions: <see cref="PlcListener.StartAsync"/> succeeded (on first attempt or
+    /// after a recovery attempt).
+    /// </summary>
+    Bound,
+
+    /// <summary>
+    /// The listener is not bound; the supervisor is waiting for the next Polly retry delay
+    /// before reattempting. Entered after any failed bind (at startup or at runtime).
+    /// </summary>
+    Recovering,
+
+    /// <summary>
+    /// Terminal state. <see cref="PlcListenerSupervisor.StopAsync"/> was called; the supervisor
+    /// task has been cancelled and will not retry.
+    /// </summary>
+    Stopped,
+}
+
+/// <summary>
+/// Immutable point-in-time snapshot of a supervisor's state. Consumed by Phase 07's
+/// status page via <see cref="PlcListenerSupervisor.Snapshot"/>.
+///
+/// <para><b>RecoveryAttempts semantics</b>: this counter <em>accumulates over the lifetime
+/// of the supervisor</em> and is never reset. Operators reading the status page should
+/// interpret it as "how many times has this listener faulted or failed to bind since
+/// the service started" — useful for detecting port-flapping or repeated OS network
+/// resets. Phase 07 surfaces it as-is.</para>
+/// </summary>
+/// <param name="State">Current state of the supervisor.</param>
+/// <param name="LastBindError">
+/// Most recent bind failure message (up to 256 chars). <c>null</c> if the listener
+/// has never failed to bind.
+/// </param>
+/// <param name="RecoveryAttempts">
+/// Total number of failed bind attempts over the lifetime of this supervisor.
+/// Accumulates; never resets to 0.
+/// </param>
+public sealed record SupervisorSnapshot(
+    SupervisorState State,
+    string? LastBindError,
+    int RecoveryAttempts);