fix(encoder): observability — silence was the bug

First real recording (2026-09-01): ffmpeg died at ~0.9s mid-session with ZERO log
lines while the pump fed a corpse for 9 more seconds. Three blind spots closed:
- OnStderrLine discarded everything non-progress: ffmpeg's actual words (startup
  config, warnings, the reason it quit) now go to AppLog prefixed 'ffmpeg:' (400-char
  cap so a \r-choked blob can't flood).
- Unexpected exit only fired ProcessFailed when code != 0 — a CLEAN exit vanished
  silently. Any exit we didn't request is now logged and fired, any code.
- ExitCode read raced StopAsync's Dispose (tonight's 'No process is associated'
  line) — guarded.
- VM output resolver logs once/5s when a webcam element resolves to a null frame
  (preview showed the cam, recording didn't — the layer skip was invisible).
FfmpegEncoderTests + FramePumpTests 21/21 against the new code, 0 warnings.

ViewModels/MainViewModel.cs

@@ -459,6 +459,26 @@ public partial class MainViewModel : ViewModelBase
     // preview: webcam by DeviceId, live captures (the background) by CaptureKey,
     // images/background by AssetId. A null frame leaves the element transparent.
     private VideoFrame? ResolveOutputFrame(SceneElement element)
+    {
+        var frame = ResolveOutputFrameCore(element);
+        // Output-side eyes (2026-09-01): preview showed the webcam but the recording
+        // didn't — a null resolver frame silently DROPS the layer. Log once per 5s
+        // per kind so the repro run tells acquire-mismatch from compose-skip.
+        if (frame == null && element is WebcamSceneConfig webcam)
+        {
+            var now = DateTime.UtcNow;
+            if (now - _lastWebcamNullLog >= TimeSpan.FromSeconds(5))
+            {
+                _lastWebcamNullLog = now;
+                AppLog.Write($"Output resolver: webcam '{webcam.WebcamId}' frame is null — layer skipped in composite");
+            }
+        }
+        return frame;
+    }
+
+    private DateTime _lastWebcamNullLog = DateTime.MinValue;
+
+    private VideoFrame? ResolveOutputFrameCore(SceneElement element)
     {
         return element switch
         {