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/Bcd/BcdValidationError.cs

@@ -0,0 +1,32 @@
+namespace Mbproxy.Bcd;
+
+/// <summary>Discriminates the class of validation failure in a resolved BCD tag list.</summary>
+public enum BcdValidationError
+{
+    /// <summary>Two or more entries share the same Modbus register address.</summary>
+    DuplicateAddress,
+
+    /// <summary>
+    /// A 32-bit entry's high register (Address+1) collides with another entry's address.
+    /// </summary>
+    OverlappingHighRegister,
+
+    /// <summary>An entry has a Width that is not 16 or 32.</summary>
+    InvalidWidth,
+}
+
+/// <summary>A hard validation failure that prevents the map from being used.</summary>
+public sealed record BcdError(BcdValidationError Kind, string Message, ushort? Address);
+
+/// <summary>A non-fatal advisory that rides along with the map.</summary>
+public sealed record BcdWarning(string Message, ushort? Address);
+
+/// <summary>
+/// Result of a <see cref="BcdTagMapBuilder.Build"/> call.
+/// When <see cref="Errors"/> is non-empty the map is partial (only valid entries are included).
+/// Callers should treat any error as a fatal configuration problem at startup.
+/// </summary>
+public sealed record ValidationResult(
+    BcdTagMap Map,
+    IReadOnlyList<BcdError> Errors,
+    IReadOnlyList<BcdWarning> Warnings);