perf(capture): slice 8 — buffer ring + paste-cache Epoch; gen2 visibility (take-11 spikes)

Take 11 (c10ce06c) validated the off-UI architecture: typical frames land
work ~10ms + wait ~6.8ms = 16.7 exactly on the deadline; 212/300 best yet.
The ENTIRE remaining gap is periodic 35-65ms render spikes that WORSENED
across the take (189 -> 147) — the signature of gen2 GC pauses. Biggest
churner is structural: the screen capture minted a fresh ~8.3MB byte[] per
DWM frame (~500MB/s of LOH), a producer OBS never does (it owns fixed
surface pools).

- ScreenCaptureFrameSource: 4-deep buffer ring with size-matched slots (a
  <=17ms consumer cannot be lapped at 60Hz) + reused downscale row scratch.
- VideoFrame.Epoch: monotonic per producer frame. The paste cache keys on
  array IDENTITY, so recycled arrays MUST be distinguished — epoch joins the
  PasteKey. Producers handing fresh arrays leave it 0 (key unchanged effect).
- Stats print 'gen2 +N' per 5s window: next take acquits or convicts GC
  without another guess (rule: prove the stage).
- Test (the ONE): PasteCache_RecycledArrayWithNewEpoch_ReRasterizes_NotStaleHits
  — same array, new content, bumped epoch; fails on the old key by
  construction. 37/37 compositor/pump, clean build.
- Next suspect if gen2 stays hot: the 10Hz WebView2 capture (full-canvas PNG
  decode + fresh arrays on the UI thread) — recorded, untouched.

Creator audio ask queued in the same working session (+40% post-mix master
gain before the -1dBFS limiter) lands as its own commit next.

HANDOFF.md

@@ -60,12 +60,24 @@ RealApp boot-smoke. Scope-check passed.
    `avg wait` so render+submit+wait ≈ period (accounting closed — nothing can hide). Webcam now
    routes through the paste cache too (bypass re-sampled 156k px even between identical device
    frames). 52/52 per-class green, clean build 0 warnings, committed this slice.
-4. **Take 11 (user, ~30s record-only):** read the superscript; expect `≈300/300 frames, wait ≈ the true remainder, worst render` now visible. If ~300: playback must be honest 1x — saga CLOSED, Unit B starts. If still ~250-280 with worst-render spikes: raster-miss spikes (web capture ~ every second) — next slice is pre-rasterizing on content change rather than on first-tick-after-change (cache the miss behind a swap-in). If wait is STILL large: the context theory was wrong and I have egg to eat — re-instrument, don't guess.
-   ~22, wait ~0-3` — honest 60fps IF render+submit ≤ ~16.7. If wait is near zero and n/300 sits at
-   ~200, the remaining gap is pure render 22ms → next slice = per-phase compositor timing (the stats
-   can split blit phases the same way they split resolve; that is the honest path, not a guess).
-   If take 10 lands at ~300/300 → recording saga CLOSED.
-5. **Unit B — the top bar + session logic (user spec 2026-09-04 re-sent twice + decisions settled in Q&A):**
+4. **Take 11 ran + slice 8 (2026-09-04, committed with the audio unit below):** off-UI loop
+   WORKED — typical frames land exactly on the deadline (work ~10 + wait ~6.8 = 16.7; 212/300).
+   Remaining gap = periodic 35-65ms spikes growing across the take = gen2 GC pauses; the capture
+   path minted a fresh ~8.3MB byte[] per DWM frame (~500MB/s LOH). Slice 8: 4-deep buffer ring
+   in ScreenCaptureFrameSource (size-matched slots, reused row scratch) + `VideoFrame.Epoch`
+   joins the paste-cache key so recycled arrays can never false-hit + `gen2 +N` per stats window
+   (suspect-or-acquit — never guess at the tail again). Test `PasteCache_RecycledArrayWithNewEpoch_
+   ReRasterizes_NotStaleHits` fails on the old key by construction. 37/37 compositor/pump green.
+4b. **AUDIO (creator ask 2026-09-04):** post-mix volume up ~40% — constant master gain applied
+   in AudioMixer.FillAndMix BEFORE the −1 dBFS limiter (limiter still owns the ceiling: the
+   gain can't add clipping, it just makes the limiter bind sooner on hot material). If the
+   creator reports pumping on loud game audio → the constant is the tuning knob (or per-input
+   defaults need raising instead — discuss with data).
+5. **Take 12 (user, ~30s record + talk):** stats: `gen2 +N` should be ~0-1/window, `worst render`
+   → ~16-20ms, n/300 → 300; playback honest speed; file audio ~40% hotter than before. If gen2
+   still >2/window: next suspect is the 10Hz WebView2 capture (full-canvas PNG decode + fresh
+   arrays on the UI thread) — throttle or move it, cited pattern first.
+6. **Unit B — the top bar + session logic (user spec 2026-09-04 re-sent twice + decisions settled in Q&A):**
    - Two-line top bar. Line 1: center = REC + **LIVE** pills (text renamed from ON-AIR; pills become
      mutually-exclusive RADIOS — record-OR-stream ruling), right = avatar + **Login/Logout** button
      (no account status light). Line 2: centered primary **Start** (grayed while NO pill armed —