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/tests/Mbproxy.Tests/Configuration/ReloadValidatorTests.cs

@@ -0,0 +1,158 @@
+using Mbproxy.Configuration;
+using Mbproxy.Options;
+using Xunit;
+
+namespace Mbproxy.Tests.Configuration;
+
+/// <summary>
+/// Unit tests for <see cref="ReloadValidator.Validate"/>.
+/// Each test covers one specific failure mode or the happy path.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ReloadValidatorTests
+{
+    // ── Helpers ───────────────────────────────────────────────────────────────────────────
+
+    private static PlcOptions MakePlc(string name, int listenPort, string host = "127.0.0.1")
+        => new() { Name = name, ListenPort = listenPort, Host = host, Port = 502 };
+
+    private static MbproxyOptions MakeOptions(
+        PlcOptions[] plcs,
+        int adminPort = 8080,
+        BcdTagListOptions? global = null)
+        => new()
+        {
+            Plcs      = plcs,
+            AdminPort = adminPort,
+            BcdTags   = global ?? new BcdTagListOptions(),
+        };
+
+    // ── 1. Duplicate PLC name → fails ────────────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_DuplicatePlcName_Fails()
+    {
+        var opts = MakeOptions([
+            MakePlc("PLC-A", 5020),
+            MakePlc("PLC-A", 5021),  // same name
+        ]);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("PLC-A") && e.Contains("uplicate"));
+    }
+
+    // ── 2. Duplicate ListenPort → fails ──────────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_DuplicateListenPort_Fails()
+    {
+        var opts = MakeOptions([
+            MakePlc("PLC-A", 5020),
+            MakePlc("PLC-B", 5020),  // same port
+        ]);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("5020") && e.Contains("uplicate"));
+    }
+
+    // ── 3. AdminPort collides with a PLC's ListenPort → fails ────────────────────────────
+
+    [Fact]
+    public void Validate_AdminPortCollidesWith_PlcListenPort_Fails()
+    {
+        var opts = MakeOptions(
+            plcs: [MakePlc("PLC-A", 5020)],
+            adminPort: 5020);  // collides with PLC-A
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("AdminPort") && e.Contains("5020"));
+    }
+
+    // ── 4. Per-PLC BCD map build error → fails ────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_PerPlc_BcdMapBuildError_Fails()
+    {
+        // A 32-bit tag at address 100 and a 16-bit tag at 101 collide on high register.
+        var global = new BcdTagListOptions
+        {
+            Global =
+            [
+                new BcdTagOptions { Address = 100, Width = 32 },
+                new BcdTagOptions { Address = 101, Width = 16 },  // overlaps 100's high register
+            ],
+        };
+        var opts = MakeOptions([MakePlc("PLC-A", 5020)], global: global);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("PLC-A"));
+    }
+
+    // ── 5. Port out of range → fails ─────────────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_PortOutOfRange_Fails()
+    {
+        // ListenPort 0 is below the valid [1, 65535] range.
+        var opts = MakeOptions([MakePlc("PLC-A", 0)]);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("0") && e.Contains("range"));
+    }
+
+    // ── 5b. AdminPort out of range → fails ───────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_AdminPortOutOfRange_Fails()
+    {
+        var opts = MakeOptions([MakePlc("PLC-A", 5020)], adminPort: 70000);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("70000") && e.Contains("range"));
+    }
+
+    // ── 6. Happy path → passes ───────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_HappyPath_Passes()
+    {
+        var global = new BcdTagListOptions
+        {
+            Global = [new BcdTagOptions { Address = 1072, Width = 16 }],
+        };
+        var opts = MakeOptions(
+            plcs: [MakePlc("PLC-A", 5020), MakePlc("PLC-B", 5021)],
+            adminPort: 8080,
+            global: global);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.True(valid);
+        Assert.Empty(errors);
+    }
+
+    // ── 7. Empty PLC name → fails ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Validate_EmptyPlcName_Fails()
+    {
+        var opts = MakeOptions([MakePlc("", 5020)]);
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("non-empty"));
+    }
+}