mbproxy: replace status page with a live SignalR web dashboard

The single auto-refreshing zero-JS status page gave operators a 25-column
wall and no way to drill into one connection. This adds a Bootstrap fleet
dashboard (filterable/sortable KPI table) and a per-PLC detail page with a
real-time debug view of raw PLC-side BCD vs. decoded client-side values,
streamed live over a SignalR feed. The debug view is fed by an on-demand
per-tag value capture, armed only while a detail page is open. All assets
(Bootstrap, SignalR client, fonts) are embedded so the UI works unchanged
on firewalled networks; GET /status.json is untouched for scrapers.

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

mbproxy/src/Mbproxy/Admin/StatusSnapshotBuilder.cs

@@ -19,17 +19,60 @@ internal sealed class StatusSnapshotBuilder
     private readonly ServiceCounters _serviceCounters;
     private readonly AssemblyVersionAccessor _version;
     private readonly ProxyWorker _proxyWorker;
+    private readonly TagCaptureRegistry _captureRegistry;
 
     public StatusSnapshotBuilder(
         IOptionsMonitor<MbproxyOptions> options,
         ServiceCounters serviceCounters,
         AssemblyVersionAccessor version,
-        ProxyWorker proxyWorker)
+        ProxyWorker proxyWorker,
+        TagCaptureRegistry captureRegistry)
     {
-        _options        = options;
+        _options         = options;
         _serviceCounters = serviceCounters;
-        _version        = version;
-        _proxyWorker    = proxyWorker;
+        _version         = version;
+        _proxyWorker     = proxyWorker;
+        _captureRegistry = captureRegistry;
+    }
+
+    /// <summary>
+    /// Builds the connection-detail debug snapshot for one PLC: the last value observed
+    /// for every configured BCD tag. Returns an empty, disarmed snapshot when
+    /// <paramref name="plcName"/> is unknown (e.g. a detail page open for a PLC removed
+    /// by hot-reload).
+    /// </summary>
+    public PlcDebugSnapshot BuildDebug(string plcName)
+    {
+        if (!_captureRegistry.TryGet(plcName, out var capture))
+            return new PlcDebugSnapshot(CaptureArmed: false, Tags: Array.Empty<TagValueDto>());
+
+        var now  = DateTimeOffset.UtcNow;
+        var tags = capture.Snapshot()
+            .Select(o => ToTagDto(o, now))
+            .ToList();
+
+        return new PlcDebugSnapshot(capture.IsArmed, tags);
+    }
+
+    private static TagValueDto ToTagDto(TagValueObservation o, DateTimeOffset now)
+    {
+        bool hasValue = o.UpdatedAtUtc.HasValue;
+
+        string rawHex = !hasValue
+            ? "—"
+            : o.Width == 32
+                ? $"0x{o.RawHigh:X4}{o.RawLow:X4}"
+                : $"0x{o.RawLow:X4}";
+
+        return new TagValueDto(
+            Address:      o.Address,
+            Width:        o.Width,
+            HasValue:     hasValue,
+            Direction:    o.Direction == CaptureDirection.Write ? "write" : "read",
+            RawHex:       rawHex,
+            DecodedValue: o.DecodedValue,
+            UpdatedAtUtc: o.UpdatedAtUtc?.ToString("o"),
+            AgeSeconds:   o.UpdatedAtUtc is { } at ? (now - at).TotalSeconds : null);
     }
 
     /// <summary>