feat(engine): drawElementImage capture service

[vanceingalls]

drawElement capture service (stack 2/6)

The core drawElementImage-based capture service — reads a DOM subtree's paint records directly into a <canvas layoutsubtree> on the macOS GPU, the mechanism that makes fast capture ~2× cheaper per frame than the CDP screenshot round-trip.

What this adds — packages/engine/src/services/drawElementService.ts (+834)

• captureDrawElementFrame — force a paint-record invalidation, await the canvas paint event, then canvas.drawElementImage(root). Paint-wait ~1.3ms/frame; encode dominates (offloaded in 4/6).
• Accelerated-canvas handling — WebGL/WebGL2/WebGPU canvases never repaint (stale paint record), so they're hidden before the paint and composited live via drawImage underneath the drawElementImage output; 2D canvases refresh natively via a per-frame sentinel paint on paint-synced hosts.
• produceDrawElementFrame / worker-encode entry — hands the captured bitmap to the OffscreenCanvas encode worker (wired in 4/6).
• Background/ancestor fill so drawElementImage(root) matches what captureScreenshot composites.

Notes

• macOS-GPU-only by design (BeginFrame path on Linux is separate); gated upstream.
• Tests: drawElementService.test.ts + drawElementService.integration.test.ts.

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Stack: #1916 → #1917 (here) → #1918 → #1919 → #1920 → #1921. Rebased onto current main; supersedes #1295 + #1444. Intermediate PRs don't compile independently — feature is 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
• feat(engine): 3D projection + compositor-effect risk gate #1918
• feat(engine): drawElementImage capture service #1917 👈 (View in Graphite)
• feat(engine,cli): drawElement fast-capture config + CLI flag #1916
• main

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