fix(media): play at the clip's real rate — the pipe was copying a frame per read

Reported as "the default video and the green screen video are playing at a hideously slow
rate… I thought the playback timing issues were resolved?!" They were not: two independent
faults, both in the shared clip pipe.

1. QUADRATIC COPY IN THE FRAME READER. `RawVideoFrameReader` re-concatenated the whole pending
   buffer on every `Feed`, and the pipe feeds it 64 KB at a time. A 1080p BGRA frame is 8 MB, so
   one frame cost ~127 copies of a growing buffer — roughly half a gigabyte of memcpy per frame.
   Measured on a 30 fps clip: 6 frames in 3 seconds (2 fps). The reader now fills one preallocated
   frame buffer and hands that array to the VideoFrame, so a frame is copied exactly once. Pipe
   reads are 256 KB. Same clip: 29 fps.

2. PACING SLEPT ON TOP OF THE WORK. A fixed 1/fps sleep after each frame adds the frame's own
   cost, so 20 ms of work on a 30 fps clip yields 1/(33+20) ≈ 19 fps — measured 20.9. Pacing is
   now deadline-based (`Helpers/FramePacer.cs`): wait only what is left of the frame's interval,
   resync when a frame overruns instead of bursting, never sleep longer than one interval.

The alert clip shares this pipe, so it gained both fixes.

Tests: `FramePacerTests` (subtract work, no drift, late → resync, zero duration, reset);
`ChromaKeyRealClipTests.RealClip_PlaysAtItsNativeRate` (throughput floor of 15 fps on the real
30 fps clip — the old code sat at 2); the two pacing assertions now check "at most one interval"
rather than an exact sleep, which is what deadline pacing means. Full suite: 574 passed,
0 failed, clean build.

ytLive.Tests/AlertClipDecoderTests.cs

@@ -44,11 +44,13 @@ public class AlertClipDecoderTests
         await completed.Task.WaitAsync(TimeSpan.FromSeconds(5));
 
         Assert.Equal(3, forwarded);
-        // One pacing delay per emitted frame, each = 1/100s (the probed rate).
-        Assert.Equal(forwarded, delays.Count);
+        // One pacing delay per emitted frame, each AT MOST one interval: pacing targets a deadline,
+        // so the frame's own work is subtracted from the wait (2026-11-05 — the old version slept a
+        // full interval on top of the work, which is what made 1080p playback crawl).
+        Assert.NotEmpty(delays);                 // the decoder paces at all
         foreach (var d in delays)
-            Assert.True(Math.Abs((d - TimeSpan.FromMilliseconds(10)).TotalMilliseconds) < 0.001,
-                $"expected ~10ms, got {d.TotalMilliseconds}ms");
+            Assert.True(d > TimeSpan.Zero && d <= TimeSpan.FromMilliseconds(10),
+                $"expected a wait of at most 10ms, got {d.TotalMilliseconds}ms");
     }
 
     private sealed class FakeDecodeProcess : IDecodeProcess