perf(chat): slice 3 — raster-on-change cache in ChatOverlayLayer (take-5 fix)

Take 5: render 58.9 -> 25.5ms (138/300, still ~2.2x). The blits were fixed; the
resolver was not: ResolveOutputFrame -> RenderChatBox ran a FULL WPF raster
(FormattedText + RenderTargetBitmap + CopyPixels + channel swap) EVERY tick
whenever the chat buffer was non-empty — and the buffer survives sessions, so
even a signed-out record-only take paid it. Established answer (OBS text
sources): re-render on change, blit the cache every tick.

ChatOverlayLayer: content version bumped from Messages.CollectionChanged
(covers adds, the 500-cap removal, the fade Clear from any caller) + a config
key (size + all Chat* appearance props); RenderFrame returns the cached
VideoFrame by identity until either changes (compositor only reads cached
frames). Conservative ordering (version latched BEFORE render) makes a
mid-render message re-render next tick, never serve stale.

ONE integration test: ChatOverlayLayerCacheTests (RealApp, real renderer):
Same() for unchanged inputs, NotSame() on message/config change, null on
empty. Full regression green (62 across touched classes), clean build 0
warnings. Accepted cost pending take 6: one ~15-25ms tick per arriving
message; if live-chat bursts sag n/300, next slice = debounced off-tick
re-render. Docs same commit: ai.md pipeline section, TASKS.md TASK 18,
MyMistakes recipe (raster-on-change + session-surviving-buffer trap),
HANDOFF (take 6 -> then Unit B, spec unchanged).

HANDOFF.md

@@ -37,12 +37,18 @@ RealApp boot-smoke. Scope-check passed.
 
 ## OPEN — next, in order
 
-1. **Take 5 (user, ~30s record-only):** read `%APPDATA%\ytLlive\startup.log` — expect
-   `FramePump stats: ≈300/300 frames per 5s, avg render ≤ ~10ms, avg submit ~1ms` + ffprobe frames ≈
-   60×duration. If render is under budget but `n/300` still short → suspect capture-side (the 1440p
-   `DownscaleBgra` per-frame alloc loop on the capture thread) or residual GC — next slice.
-   Also for the user to eyeball: webcam now at true 60fps, audio still in sync.
-2. **Unit B — the top bar + session logic (user spec 2026-09-04, decisions settled):**
+1. **Take 5 happened (2026-09-04 10:49):** `138/300 frames per 5s, avg render 25.5ms` — blits
+   fixed but the resolver's `RenderChatBox` full-rasterized the chat box EVERY tick whenever the
+   message buffer was non-empty (the buffer survives sessions — a signed-out record-only take paid
+   chat render cost!). **Slice 3 shipped same day:** `ChatOverlayLayer` content-versioned cache —
+   raster on message/config change, blit the cached frame every tick (OBS text-source pattern).
+   Tests: `ChatOverlayLayerCacheTests` (the ONE, RealApp) + full regression green (62 across
+   touched classes), clean build 0 warnings.
+2. **Take 6 (user, ~30s record-only):** expect `≈300/300 frames per 5s, avg render ≤ ~10ms`. If
+   chat-burst drops one frame per message (avg fine but `n/300` sags during live chat): debounced
+   off-tick re-render is the next slice. If steady at ~300 with render <10 → recording saga
+   CLOSED, go Unit B (below).
+3. **Unit B — the top bar + session logic (user spec 2026-09-04, decisions settled):**
    - 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 —
@@ -59,7 +65,7 @@ RealApp boot-smoke. Scope-check passed.
    - LIVE+Start → **Go-Live dialog stays** as preflight: prefilled from Text-drawer `Broadcast.*`,
      unfilled fields visibly prompted, explicit confirm → `PrepareAndStartLiveAsync` (user: "going
      live is scary — confirmation allows back-out + testing up to go-live").
-   - Bottom-bar metrics init/maintain: `ResetHealth` + `HealthUpdated` exist — verify on take 5/6.
+   - Bottom-bar metrics init/maintain: `ResetHealth` + `HealthUpdated` exist — verify on take 6.
    - F6 "start/end" hotkey routes through `HandleHotkey` — check it honors the new grayed-Start gate.
    - ONE integration test (hermetic): pills↔button state machine + record-path seam (an
      `internal static Func<SaveFileDialog-ish prompt>` override seam mirroring `RegistrarOverride`