* fix(engine): commit render-frame siblings with a visual BeginFrame at init
Chunk-lambda renders drop a periodic near-black frame — one every
chunk_frames/worker_count frames (every 60 on a 4-worker single-video chunk),
YAVG ~22 against YMAX ~240 in signalstats. Local single-process renders don't
show it because they don't run under BeginFrame.
It's the isNewImage branch in injectVideoFramesBatch: the first time a session
paints a given videoId there's no __render_frame__ sibling yet, so it creates
the <img> on the spot (createElement + insertBefore) right before capture.
Under HeadlessExperimental.BeginFrame the compositor doesn't have that fresh
layer in the immediately-next frame, so the first captured frame per session
paints only body background + already-composited overlays. Each lambda worker
is its own session, hence the worker-boundary periodicity.
Pre-create the hidden sibling at the end of initializeSession, then drive one
non-capture visual BeginFrame (noDisplayUpdates: false) to composite the new
layers before the first real capture. The warmup ticks are noDisplayUpdates:
true (they advance the clock but don't paint) and the per-frame seek doesn't
tick, so this explicit visual frame is what actually commits the layers; its
tick sits in the gap between warmup and frame 0 so ticks stay monotonic and no
render frame is consumed. Every subsequent inject then takes the hasImg=true
(src-update) path; the isNewImage branch stays as a fallback for callers that
don't go through initializeSession.
* fix(engine): place the render-frame commit tick before the liveness probe
The commit tick at init sends its BeginFrame at `beginFrameTimeTicks - 1·interval`.
The producer's liveness probe then fires right after init at
`beginFrameTimeTicks - 5·interval` — an earlier tick. Per-session BeginFrame time
has to be monotonic, so the probe running backwards past the commit tick stalls
chrome-headless-shell indefinitely; the engine reads that timeout as a SwiftShader
heavy-layer stall and routes the render to screenshot capture, which then dies
relaunching and hangs the shard to the job timeout.
Reproduced on a native x86 SwiftShader host and bisected: with the commit tick
present the probe times out even with zero render-frame siblings created, so it's
the tick ordering, not layer count. Moving the commit tick to `-6·interval` (below
the probe, above the warmup ticks) keeps warmup < commit < probe < capture
monotonic and clears the stall on every affected comp — sub-composition-video,
chat, style-5-prod — while a healthy comp (style-18-prod) is unchanged. The commit
tick itself is untouched, so the black-frame fix it exists for still holds.
