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

[vanceingalls]

Frame-capture core — routing, worker-encode, static-dedup unification (stack 4/6)

The heart of the feature. Wires drawElement capture (2/6) + the risk gate (3/6) into the capture loop, adds worker-offloaded encode, and unifies our static-frame dedup with the implementation main shipped independently.

packages/engine/src/services/frameCapture.ts (+714 / −33)

• Capture-mode routing — screenshot / BeginFrame / drawElement selection + the gate chain (video, stacked-fade, 3D, detectCssEffectRisk) that routes incompatible comps to the screenshot baseline.
• Worker-encode pipeline — depth-2 pipeline handing drawElement frames to the OffscreenCanvas encode worker (produceDrawElementFrame); frees the main thread so capture (paint-wait + readback), not encode, is the throughput bound.
• Per-frame screenshot fallback — a drawElement frame that fails (No cached paint record, clip-cut boundary) falls back to a screenshot for that frame instead of aborting the render.

⚠️ Static-dedup unification (reviewer attention)

main and this branch each independently implemented static-frame dedup with the same symbol names. This PR keeps main's implementation as canonical (default-on config.staticFrameDedup, clip-start safety, density-verify backstop, observability) and extends it to the drawElement + worker-encode path — worker-path frame reuse (lastEncodeResult) + blank-frame detection (deFrameSizes), armed on the drawElement-proceed site (pre-canvas-injection, so main's screenshot-based density-verify remains a valid backstop for the DE path). Our opt-in HF_STATIC_DEDUP env is dropped — superseded by main's single default-on config switch governing both capture modes. Net behavior change: static-dedup is now default-on for both screenshot and drawElement paths.

Also: browserManager.ts, screenshotService.ts, index.ts (exports/glue).

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Stack: #1916 → #1917 → #1918 → #1919 (here) → #1920 → #1921. Rebased onto current main; supersedes #1295 + #1444. Intermediate PRs don't compile independently — feature green at the tip (#1921), tsc-clean + 231 tests pass.

🤖 Generated with Claude Code

[vanceingalls]

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• perf(engine,producer): batch N drawElement frames per CDP round-trip (HF_DE_BATCH) #1928
• feat(lint,player): fast-capture lint rule + player media sync #1921
• feat(producer): fast-capture render stages + remote bg-image localizer #1920
• feat(engine): frame-capture core — fast-capture routing, worker-encode, dedup extension #1919 👈 (View in Graphite)
• feat(engine): 3D projection + compositor-effect risk gate #1918
• feat(engine): drawElementImage capture service #1917
• feat(engine,cli): drawElement fast-capture config + CLI flag #1916
• main

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