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.

ytLive.Tests/SceneCompositorTests.cs

@@ -303,6 +303,46 @@ public class SceneCompositorTests
         Assert.Equal(build.BgraPixels, hit.BgraPixels);
     }
 
+    /// <summary>2026-09-12 regression — the webcam gray-block bug: the CropBounds
+    /// Fill-scaling work (ed9d7c1, same day as the transparency fix) added
+    /// cbX/cbY/cbW/cbH but only assigned them inside the CropBounds-present branch;
+    /// every CropBounds-less source (webcam, images — anything but the web widget)
+    /// fell through with cbW=cbH=0, so sxCrop/syCrop (= norm * cbW/cbH) were always
+    /// 0 and EVERY destination pixel sampled source (0,0) — a whole scaled webcam
+    /// collapsed to one corner pixel's color. All rect-placed elements route through
+    /// this exact sampler via the paste cache (take-9), so this is not webcam-specific;
+    /// a two-color source proves distinct destination regions read distinct source
+    /// regions instead of a single flat pixel.</summary>
+    [Fact]
+    public void BlitContentRaw_NoCropBounds_SamplesAcrossFullSource_NotJustOrigin()
+    {
+        var source = Split(64, 32, 255, 0, 0, 0, 0, 255); // left=red, right=blue, no CropBounds
+        var background = new Source { Type = SourceType.DisplayCapture, IsBackground = true, CaptureKey = "monitor:0" };
+        var webcam = new WebcamSceneConfig { X = 0, Y = 0, Width = 40, Height = 20 }; // scaled (not 1:1)
+        var scene = new Scene { Name = "Live" };
+        scene.Elements.Add(background);
+        scene.Elements.Add(webcam);
+
+        VideoFrame? FrameFor(SceneElement e) => e switch
+        {
+            Source { IsBackground: true } => Solid(128, 64, 0, 0, 0),
+            WebcamSceneConfig => source,
+            _ => null,
+        };
+
+        var options = new CompositorOptions
+        {
+            SourceRectX = 0, SourceRectY = 0, SourceRectWidth = 128, SourceRectHeight = 64,
+            OutputWidth = 128, OutputHeight = 64,
+        };
+
+        var compositor = new SceneCompositor();
+        var output = compositor.Render(scene, FrameFor, null, options);
+
+        AssertColor(output, 5, 10, 255, 0, 0);  // near the left edge of the webcam rect: source's red half
+        AssertColor(output, 35, 10, 0, 0, 255); // near the right edge: source's blue half — fails (reads red) on the bug
+    }
+
     /// <summary>take-16 regression gate — GC churn convicted mechanically: a ring-style
     /// producer hands out a small set of rotating arrays with a NEW EPOCH every frame
     /// (exactly what the capture/camera rings do). The compositor must re-rasterize INTO