feat(producer): fast-capture render stages + remote bg-image localizer

[vanceingalls]

Producer render stages + remote bg-image localizer (stack 5/6)

Producer-side wiring of fast capture into the render pipeline, plus a cross-origin asset fix.

Render stages

• captureStreamingStage.ts (+102/−) — branch to the worker-encode pipeline when enabled, else the standard capture loop; normalized ensureFrameWritten to main's arg shape.
• compileStage.ts, captureStage.ts, probeStage.ts — fast-capture routing, BeginFrame-liveness probe, gate wiring.
• renderOrchestrator.ts, regression-harness.ts, stubs/hf-early-stub.ts (+191) — orchestration glue + the injected early-stub that registers seek/timeline hooks; generated/hf-early-stub-inline.ts regenerated from the stub.

htmlCompiler.ts (+93) — localizeRemoteBackgroundImages

Fix for cross-origin damage: drawElementImage omits cross-origin content (spec: keeps the canvas readable), so a remote CSS background-image: url(https://…) renders black on the DE path while the screenshot baseline shows it. This scans background(-image): url(remote), downloads + rewrites to same-origin (reusing the font-face url() machinery), wired between localizeRemoteImageSources and localizeRemoteFontFaces. (The prior localizer covered <img>/<video>/@font-face but not CSS backgrounds.)

Tests / fixtures

htmlCompiler.test.ts (localizer), validate-fast-video.ts script, and fast-capture-gsap / fast-capture-3d / css-spinner / render-symlinked-assets fixtures.

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Stack: #1916 → #1917 → #1918 → #1919 → #1920 (here) → #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 👈 (View in Graphite)
• 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
• feat(engine,cli): drawElement fast-capture config + CLI flag #1916
• main

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