perf(pump): slice 6 — break the 15.6ms sleep quantum (the REAL ceiling behind takes 7-9)

Take 9's numbers were decisive: paste cache moved work to ~25ms/frame but the
period stayed ~37ms. The missing ~12ms per tick is Task.Delay rounding every
sub-tick request up to the Windows system-clock tick (~15.6ms default —
documented: learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.delay).
A frame finishing 3ms early requested 3ms and slept 15.6. Producer capped at
~27fps no matter how fast the compositor got — which is why two real render
fixes read as 'zero change' in playback. Game-loop/OBS canon for this
(stackoverflow.com/questions/5441464; learn.microsoft.com/en-us/windows/win32/
api/timeapi/nf-timeapi-timebeginperiod): raise the timer resolution for the
session, sleep only the bulk of the remainder, and SPIN the last ~2ms across
the deadline.

- FramePump: timeBeginPeriod(1) on entering the pump loop, timeEndPeriod(1) in
  the finally; pacing = bulk _pacingDelay(ahead - 2ms) + bounded Thread.SpinWait
  tail; blown deadlines rebase unchanged (never burst).
- Stats now report avg wait: render+submit+wait must equal the period — the
  accounting is closed, no stage can hide in an unmeasured gap again.
- Webcam dropped its IsOpaque paste-cache bypass: it re-sampled ~156k px every
  tick even between identical device frames; cached paste beats the sampler on
  hits, costs the same on misses.

52/52 per-class green (pacing + pixel suites unchanged — output byte-stable),
clean build 0 warnings. Docs same commit (ai.md slice 6, TASKS.md take-10
gate, MyMistakes #3 + renumber, HANDOFF). User's top-bar spec remains next in
queue (Unit B) — re-sent many times, captured, no open questions.

Services/Compositor/SceneCompositor.cs

@@ -253,15 +253,12 @@ public sealed class SceneCompositor
         var ew = (float)element.Width;
         var eh = (float)element.Height;
         var isRound = element.ClipShape == ClipShape.Round;
-        if (frame.IsOpaque && element.Opacity >= 1f)
-        {
-            // Opaque full-strength layer (the live backdrop): raster it straight, no alpha path.
-            BlitContent(buffer, cropW, cropH, ex, ey, ew, eh, frame, 1f, isRound, element.IsMirrored);
-        }
-        else
-        {
-            BlitCachedLayer(buffer, cropW, cropH, ex, ey, ew, eh, frame, element, isRound);
-        }
+        // All rect-placed elements route through the paste cache (take-9 lesson): the
+        // IsOpaque bypass kept re-sampling the webcam (~156k px) EVERY tick even when
+        // the device frame had not changed; an opaque source rasterizes to an opaque
+        // raster whose paste is a per-pixel copy branch, so hits beat the sampler and
+        // misses cost exactly what the bypass cost.
+        BlitCachedLayer(buffer, cropW, cropH, ex, ey, ew, eh, frame, element, isRound);
         if (element.HasBorder)
             DrawBorder(buffer, cropW, cropH, ex, ey, ew, eh, element, isRound);
     }