docs(engine): ship the fast-capture-limitations.md every gate points at

[miga-heygen]

Summary

PRINFRA-301 reported that detectCssEffectRisk's drop-shadow fallback (correctly, still) routes to screenshot capture with no shipped fast-capture-limitations.md doc. Investigation found the gap is broader than that one gate: drawElementService.ts, frameCapture.ts, compileStage.ts, fallbackCaptureProfile.ts, validate-fast-video.ts, and the fast-video CI workflow all cite docs/fast-capture-limitations.md — several by a specific Lim N number — but the file never existed anywhere in the repo, and even if it had, packages/engine's files field didn't ship a docs/ directory in the published npm package.

• Add packages/engine/docs/fast-capture-limitations.md, covering every fallback gate currently in the engine (SwiftShader, video capture, unsupported CSS effects, 3D contexts, supersampling, clip-cut boundaries, the at-risk-timeline predictor) plus the operational requirements (Chrome build support, ffmpeg's psnr filter) and the per-frame recoverable-error fallback — cross-checked against the actual gate implementations, not just the doc pointers, so the limitation numbers match every existing Lim N citation.
• Add "docs" to @hyperframes/engine's package.json files array so the doc actually publishes.
• One incidental finding surfaced along the way, noted in the doc rather than fixed here since it touches CI semantics: .github/workflows/fast-video-validation.yml and validate-fast-video.ts's own comments ("fast capture cannot capture video on any platform yet") predate the Chrome-151 fix that removed the video-forces-screenshot gate, and haven't been re-validated since — likely stale, same flavor as the vault correction PRINFRA-301's repro already made for the drop-shadow-removed claim, but out of scope for this PR.

Test plan

• New regression test (fastCaptureLimitationsDoc.test.ts) asserts the doc exists and still defines every limitation number cited by name elsewhere in the codebase — confirmed RED without the doc (isolated via tagged git stash), GREEN with it.
• bunx vitest run packages/engine/src/services/fastCaptureLimitationsDoc.test.ts — 2/2 passed.
• bunx tsc --noEmit -p packages/engine — clean.
• bunx oxlint / bunx oxfmt --write — clean.
• bunx fallow audit --base origin/main --fail-on-issues — clean (0 issues in 3 changed files).
• Pre-commit hooks (lefthook: lint/format/fallow/typecheck/commitlint) all passed.

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