feat(engine): frame-capture core — fast-capture routing, worker-encode, dedup extension (#1919)

* feat(engine): drawElementImage capture service

* chore(ci): ignore drawElementService exports pending upstack consumers

Fallow's per-PR audit diffs against the merge base, so the bottom of the
fast-capture stack (#1917) sees drawElementService's exports as unused —
their consumers (frameCapture) land in #1919, two PRs upstack. ignoreExports
entry documents this and can be dropped once #1919 merges.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(engine): 3D projection + compositor-effect risk gate

* fix(engine): gate filter drop-shadow wherever blur gates (review)

detectCssEffectRisk documented drop-shadow as a ~29dB damage case but only
detected blur( in its three scan paths — a drop-shadow comp stayed on the
fast path despite the gate's own correctness contract. Detect drop-shadow(
in computed styles, stylesheet rules, and GSAP tween vars, pinned by a
focused test that runs the real page-side closure against a DOM shim
(computed / stylesheet / tween coverage + blur regression + effect-free
null).

Addresses miguel-heygen's blocker on #1918.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(engine): frame-capture core — fast-capture routing, worker-encode, dedup extension

# Conflicts:
#	packages/engine/src/services/screenshotService.ts

* fix(engine): document HF_FORCE_DRAWELEMENT as diagnostic-only; make armStaticDedup idempotent (review)

Addresses miguel-heygen's blockers on #1919:

- HF_FORCE_DRAWELEMENT promoted from a stale "SCRATCH/Uncommitted" comment to
  a documented diagnostic flag: it exists for upstream-Chromium repro work
  (gate-vs-API isolation, crbug 521861819 149-vs-151) and R&D on gated effect
  classes; renders under it may be damaged BY DESIGN since it bypasses gates
  whose thresholds encode measured damage. Never production; the safety-net
  blank guard also stands down under it so diagnostic frames arrive unmodified.
- armStaticDedup is now idempotent: the drawElement init path arms dedup
  before canvas injection, then initializeSession called it again — the
  second run overwrote the armed state with skipReason="capture_mode"
  (captureMode is "drawelement" by then), producing contradictory telemetry
  (armed frames + a skip reason), and re-ran the verification seeks on the
  fallback path. It now no-ops once staticFrames or a skip decision exists.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/engine/src/index.ts

@@ -82,6 +82,8 @@ export {
   closeCaptureSession,
   captureFrame,
   captureFrameToBuffer,
+  captureFrameToBufferPipelined,
+  writeCapturedFrame,
   discardWarmupCapture,
   getCompositionDuration,
   getCapturePerfSummary,
@@ -101,6 +103,7 @@ export {
   injectVideoFramesBatch,
   syncVideoFrameVisibility,
   cdpSessionCache,
+  probeBeginFrameLiveness,
   initTransparentBackground,
   captureAlphaPng,
   applyDomLayerMask,