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/Admin/StatusDto.cs

@@ -0,0 +1,106 @@
+using System.Text.Json.Serialization;
+
+namespace Mbproxy.Admin;
+
+// ── Wire DTOs for GET /status.json ───────────────────────────────────────────
+// Field names must match design.md "Status page" tables EXACTLY (camelCase via
+// JsonKnownNamingPolicy.CamelCase on the source-gen context).
+
+/// <summary>
+/// Top-level response envelope for <c>GET /status.json</c>.
+/// </summary>
+public sealed record StatusResponse(
+    ServiceFields Service,
+    ListenersAggregate Listeners,
+    IReadOnlyList<PlcStatus> Plcs);
+
+/// <summary>Service-wide identity and reload counters.</summary>
+public sealed record ServiceFields(
+    long UptimeSeconds,
+    string Version,
+    DateTimeOffset? ConfigLastReloadUtc,
+    int ConfigReloadCount,
+    int ConfigReloadRejectedCount);
+
+/// <summary>Aggregate listener state across all configured PLCs.</summary>
+public sealed record ListenersAggregate(int Bound, int Configured);
+
+/// <summary>Per-PLC status row.</summary>
+public sealed record PlcStatus(
+    string Name,
+    string Host,
+    int ListenPort,
+    PlcListenerStatus Listener,
+    PlcClientsStatus Clients,
+    PlcPdusStatus Pdus,
+    PlcBackendStatus Backend,
+    PlcBytesStatus Bytes);
+
+/// <summary>Listener state sub-object.</summary>
+public sealed record PlcListenerStatus(
+    string State,
+    string? LastBindError,
+    int RecoveryAttempts);
+
+/// <summary>Connected-clients sub-object.</summary>
+public sealed record PlcClientsStatus(
+    int Connected,
+    IReadOnlyList<ClientSnapshot> RemoteEndpoints);
+
+/// <summary>Per-connection-pair snapshot for the status page.</summary>
+public sealed record ClientSnapshot(
+    string Remote,
+    DateTimeOffset ConnectedAtUtc,
+    long PdusForwarded);
+
+/// <summary>PDU counters sub-object.</summary>
+public sealed record PlcPdusStatus(
+    long Forwarded,
+    FcCounts ByFc,
+    long RewrittenSlots,
+    long PartialBcdWarnings);
+
+/// <summary>Per-function-code request counts.</summary>
+public sealed record FcCounts(
+    long Fc03,
+    long Fc04,
+    long Fc06,
+    long Fc16,
+    long Other);
+
+/// <summary>
+/// Backend connect, exception, and multiplexer telemetry. Phase 9 added
+/// <c>InFlight</c>, <c>MaxInFlight</c>, <c>TxIdWraps</c>, <c>DisconnectCascades</c>, and
+/// <c>QueueDepth</c> to surface the live state of the per-PLC TxId-multiplexed connection.
+/// </summary>
+public sealed record PlcBackendStatus(
+    long ConnectsSuccess,
+    long ConnectsFailed,
+    ExceptionCounts ExceptionsByCode,
+    double LastRoundTripMs,
+    long InFlight,
+    long MaxInFlight,
+    long TxIdWraps,
+    long DisconnectCascades,
+    long QueueDepth);
+
+/// <summary>Modbus exception counts by code.</summary>
+public sealed record ExceptionCounts(
+    long Code01,
+    long Code02,
+    long Code03,
+    long Code04);
+
+/// <summary>Byte-transfer counters.</summary>
+public sealed record PlcBytesStatus(
+    long UpstreamIn,
+    long UpstreamOut);
+
+// ── Source-generation context ─────────────────────────────────────────────────
+// TreatWarningsAsErrors is on, so the context must include every reachable type.
+
+[JsonSerializable(typeof(StatusResponse))]
+[JsonSourceGenerationOptions(
+    WriteIndented = false,
+    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
+internal partial class StatusJsonContext : JsonSerializerContext;