fix: audio silence (idempotent sync-delay configure), webcam gray block, truncated videos

- AudioSyncDelay.Configure reallocated/zeroed its buffer every ~10ms tick
  (AudioMixer re-reads the UI setting each mix), so any non-zero sync offset
  erased the just-written audio -> total silence. Now early-returns when the
  delay samples are unchanged. Regression test proven both ways.
- SceneCompositor.BlitContentRaw defaulted cbW/cbH=0 when CropBounds is null
  (regression from ed9d7c1) -> webcam blit to an empty rect = gray block.
  Default to src.Width/Height. Regression test proven both ways.
- Truncated recordings: rawvideo mux stamps frames at declared 60fps by
  arrival; a scene whose first layer is dynamic (hidden elements still count)
  kills the bake cache -> full render ~35ms -> ~27fps submitted -> halved
  file length. Static scenes bake once (246ms cold, then <1ms) -> 60fps,
  full-length (probe + 13:53 take, 301/300 per 5s, 12.46s file from 12.3s
  wall). FramePump.ProbeRender names the hot render path on slow frames.

Services/Encoder/FramePump.cs

@@ -87,6 +87,11 @@ public sealed class FramePump : IDisposable
     private bool _started;
     private long _outputIndex;
 
+    /// <summary>Captured by <see cref="ProbeRender"/> when a render exceeds ~20ms —
+    /// the 2026-09-12 half-speed-render probe. Appended (once) to the next stats or
+    /// stall line, then cleared.</summary>
+    private string _renderDetail = "";
+
     /// <summary>Forwards the encoder's parsed health — ship step 6 binds this to the bottom bar.</summary>
     public event EventHandler<StreamHealth>? HealthUpdated;
 
@@ -306,8 +311,10 @@ public sealed class FramePump : IDisposable
             IFfmpegEncoder? encoder;
             lock (_gate) encoder = _encoder;
             var dropped = encoder == null ? 0 : encoder.DroppedFrames;
+            var detail = _renderDetail;
+            _renderDetail = "";
             _log?.Invoke(statFrames == 0
-                ? "FramePump stats: NO frames produced in 5s (loop stalled?)"
+                ? "FramePump stats: NO frames produced in 5s (loop stalled?)" + (detail.Length > 0 ? " | " + detail : "")
                 : $"FramePump stats: {statFrames}/{target:F0} frames per 5s, " +
                   $"avg render {renderTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms " +
                   $"(resolve {resolveTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}), " +
@@ -315,7 +322,7 @@ public sealed class FramePump : IDisposable
                   + $"avg wait {waitTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms, "
                   + $"worst render {worstRender / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F1}ms, "
                   + $"worst submit {worstSubmit / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F1}ms, "
-                  + $"dropped {dropped}, stalls {stalls}");
+                  + $"dropped {dropped}, stalls {stalls}" + (detail.Length > 0 ? " | " + detail : ""));
             worstRender = 0;
             worstSubmit = 0;
             renderTicks = submitTicks = resolveTicks = waitTicks = 0;
@@ -454,10 +461,13 @@ public sealed class FramePump : IDisposable
                     if (iterWall > 2 * intervalTicks)
                     {
                         stalls++;
+                        var detail = _renderDetail;
+                        _renderDetail = "";
                         _log?.Invoke($"FramePump stall: iteration {iterWall / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F0}ms " +
                             $"(> 2× the {interval.TotalMilliseconds:F0}ms interval): worst render " +
                             $"{worstRender / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F0}ms, worst submit " +
-                            $"{worstSubmit / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F0}ms, dropped {encoder.DroppedFrames}");
+                            $"{worstSubmit / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F0}ms, dropped {encoder.DroppedFrames}" +
+                            (detail.Length > 0 ? " | " + detail : ""));
                     }
                     ReportStats();
                 }
@@ -505,30 +515,59 @@ public sealed class FramePump : IDisposable
         byte[]? scratch = null,
         Func<SceneElement, VideoFrame?>? resolver = null)
     {
-        // per-tick composites use the (timed) resolver; the rare bake uses the raw one
-        // so bake cost lands in "render" but not "resolve".
         resolver ??= _frameResolver;
         if (_sceneGraph == null)
-            return _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
+        {
+            var sw = System.Diagnostics.Stopwatch.StartNew();
+            var frame = _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
+            ProbeRender("full-no-graph", scene, -1, sw.ElapsedMilliseconds, 0);
+            return frame;
+        }
 
         var split = _sceneGraph.GetSplitPoint(scene);
         if (split == scene.Elements.Count)
         {
             // Fully static scene: bake once, reuse.
+            var sw = System.Diagnostics.Stopwatch.StartNew();
             var baked = _sceneGraph.GetBakedBase(scene, _frameResolver, _compositorOptions);
             if (baked != null)
-                return StretchMath.BilinearScale(baked, options.OutputWidth, options.OutputHeight);
+            {
+                var stretched = StretchMath.BilinearScale(baked, options.OutputWidth, options.OutputHeight);
+                ProbeRender("fully-static", scene, split, sw.ElapsedMilliseconds, (int)sw.ElapsedMilliseconds);
+                return stretched;
+            }
         }
 
+        var swBase = System.Diagnostics.Stopwatch.StartNew();
         var baseFrame = _sceneGraph.GetBakedBase(scene, _frameResolver, _compositorOptions);
         if (baseFrame != null)
         {
-            return SceneCompositor.CompositeLayers(
+            var frame = SceneCompositor.CompositeLayers(
                 baseFrame, scene, split, resolver, options, socialBarFrame, socialBarTop, scratch: scratch);
+            ProbeRender("base+layers", scene, split, swBase.ElapsedMilliseconds, 0);
+            return frame;
         }
 
         // No static base (first layer is dynamic or empty scene) — full render.
-        return _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
+        var swFull = System.Diagnostics.Stopwatch.StartNew();
+        var frame2 = _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
+        ProbeRender("full-render", scene, split, swFull.ElapsedMilliseconds, 0);
+        return frame2;
+    }
+
+    /// <summary>2026-09-12 half-speed-render probe: the pump only produces ~27fps
+    /// (render ~35ms vs the 16.7ms deadline), and rawvideo muxes at the DECLARED fps,
+    /// so every take muxes at ~half its wall length (the truncation complaint). This
+    /// names WHICH compositor path ate the slow frame so the fix targets the real
+    /// stage. Records only when a frame takes ≥20ms (or carries the previous detail).</summary>
+    private void ProbeRender(string path, Scene scene, int split, long totalMs, int bakeMs)
+    {
+        if (totalMs < 20 && _renderDetail.Length == 0) return;
+        var dynamics = 0;
+        foreach (var e in scene.Elements)
+            if (e.Kind == ElementKind.Dynamic) dynamics++;
+        _renderDetail = $"render={path} split={split} elements={scene.Elements.Count} dynamic={dynamics}" +
+                        (bakeMs > 0 ? $" bakeMs={bakeMs}" : "") + $" totalMs={totalMs}";
     }
 
     // Dot-matrix digits (5×7, one row per raster line, '1' = lit) burned into the