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

[vanceingalls]

3D projection + compositor-effect risk gate (stack 3/6)

packages/engine/src/services/threeDProjection.ts (+1026) — the routing intelligence that decides when a composition is safe for drawElementImage and when it must fall back to the screenshot path.

What this adds

• detectCssEffectRisk — scans the composition for compositor-only effects drawElementImage cannot reproduce and routes them to the screenshot baseline: backdrop-filter (samples the compositor backdrop), filter: blur (paint-record vs compositor inconsistency), mix-blend-mode, and animated WebGL contexts. Implemented seek-free — computed styles at t0 + stylesheet-rule scan + tween-var scan + WebGL registry scan. (This is the rewrite that fixed the earlier bug where scrubbing __hf.seek for detection corrupted GSAP .from()/overlap start-value caches for every fast-capture comp.)
• WebGL 3D projection — static-content 3D reprojection support under the fast path; animated 3D subtrees fall back.
• Presence-gate helpers for compositor-incompatible tweens.

Notes

• Pure detection/projection module; consumed by the capture core in 4/6.
• Bias is conservative — any ambiguous effect routes to screenshot (correctness over speed).

---

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

🤖 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
• feat(engine): 3D projection + compositor-effect risk gate #1918 👈 (View in Graphite)
• feat(engine): drawElementImage capture service #1917
• feat(engine,cli): drawElement fast-capture config + CLI flag #1916
• main

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