mbproxy: fix Multiplexing N6 — cache-hit replay preserves observation age

The response-cache hit path replayed captured tag observations into the
debug-view capture stamped 'now', making a cache-served read look freshly
read when its value is actually up to CacheTtlMs old. Record() now takes
an optional observedAtUtc; the cache-hit replay passes each observation's
original timestamp so the debug view shows the true age.

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

mbproxy/src/Mbproxy/Proxy/Multiplexing/PlcMultiplexer.cs

@@ -913,17 +913,18 @@ internal sealed class PlcMultiplexer : IAsyncDisposable, IMultiplexCountersProvi
 
                     // A cache hit bypasses the BCD pipeline, so the debug-view capture
                     // would otherwise never see cache-served reads. Replay the
-                    // observations captured when this entry was stored — re-stamped now,
-                    // since the client receives this value right now — so the detail
-                    // page reflects what the client actually gets. Entries stored while
-                    // no viewer was armed carry no observations; those tags self-heal on
-                    // the next cache miss.
+                    // observations captured when this entry was stored, preserving each
+                    // observation's ORIGINAL timestamp — the value the client receives is
+                    // cache-aged (up to CacheTtlMs old), so the debug view must show its
+                    // true age, not a misleading "just now". Entries stored while no
+                    // viewer was armed carry no observations; those tags self-heal on the
+                    // next cache miss.
                     if (_ctx.Capture is { IsArmed: true } capture &&
                         cached.CapturedTags is { Count: > 0 } cachedTags)
                     {
                         foreach (var obs in cachedTags)
                             capture.Record(obs.Address, obs.RawLow, obs.RawHigh,
-                                obs.DecodedValue, CaptureDirection.Read);
+                                obs.DecodedValue, CaptureDirection.Read, obs.UpdatedAtUtc);
                     }
 
                     byte[] hitFrame = BuildCacheHitFrame(originalTxId, unitId, cached.PduBytes);

mbproxy/src/Mbproxy/Proxy/TagValueCapture.cs

@@ -126,7 +126,15 @@ internal sealed class TagValueCapture
     /// <param name="rawLow">BCD-encoded low word as it sits on the PLC wire.</param>
     /// <param name="rawHigh">BCD-encoded high word (0 for a 16-bit tag).</param>
     /// <param name="decoded">Decoded binary integer the client reads/wrote.</param>
-    public void Record(ushort address, ushort rawLow, ushort rawHigh, int decoded, CaptureDirection direction)
+    /// <param name="observedAtUtc">
+    /// When the value was actually observed from the PLC. Defaults to "now" for a live
+    /// pipeline observation. The response-cache hit path passes the cached observation's
+    /// original timestamp so a cache-served read shows its true age in the debug view
+    /// rather than appearing freshly read.
+    /// </param>
+    public void Record(
+        ushort address, ushort rawLow, ushort rawHigh, int decoded, CaptureDirection direction,
+        DateTimeOffset? observedAtUtc = null)
     {
         if (!_armed)
             return;
@@ -137,7 +145,7 @@ internal sealed class TagValueCapture
             ref _slots[idx],
             new TagValueObservation(
                 _addresses[idx], _widths[idx], _names[idx], rawLow, rawHigh, decoded, direction,
-                DateTimeOffset.UtcNow));
+                observedAtUtc ?? DateTimeOffset.UtcNow));
 
         // A concurrent Disarm() may have flipped _armed (and cleared the slots) between
         // the _armed check above and the write just made — which would strand a stale