Bound figure extraction canvas memory and support cancellation

[enu3379]

Background

The vendored fig-extract engine keeps a scale-2.2 full-page canvas on every detected EngineFigure, and the viewer additionally creates preview image data. A figure-heavy paper can therefore retain hundreds of MB. In-flight scans also continue after the document changes.

Acceptance criteria

• Full-page canvases are not retained for every figure-bearing page after previews are produced
• Peak retained page render surfaces are bounded, aligned with the implementation plan's small LRU policy
• Document replacement cancels or cooperatively stops the previous extraction
• Preview Blob/Object URLs are revoked on document replacement and teardown
• A long, figure-heavy PDF has a documented memory QA result
• Engine changes are made in the upstream figure-preview-test source first, then vendored byte-identically with a version bump
• npm run typecheck, npm test, and npm run build pass

Blocker

The documented upstream is https://github.com/onetwothr1/PDFViewer-Figure-Extract. The current GitHub account and local workspace cannot access it (gh repo view and git ls-remote both report that the repository was not found). Access to that source repository, or a new upstream engine release/commit from its owner, is required before changing the vendored file. AI-assisted.

[onetwothr1]

I'll take a look and work on this.

[onetwothr1]

엔진 측 메모리 대응 완료 — vendored via #23

figure-preview-test 엔진 v2.5.1로 근본 원인을 수정했습니다 (#23). 이 이슈가 지목한 상주 구조를 제거했습니다.

체크 반영 (#23에 담긴 완료분):

• Full-page canvases not retained — cropCanvas 대체
• Engine changes upstream-first + byte-identical vendored + version bump — v2.5.1
• typecheck · test(25) · build 통과 (chore: bump fig-extract to v2.5.1 #23 브랜치 검증)

남은 항목 — Margin 측 후속 (별도 PR, #17 머지 후):

• Peak bounded (LRU) — 엔진 피크는 1장으로 bound됨. render-region.ts의 페이지 렌더 LRU 3장(implementation-plan 약속분)은 미구현.
• Document replacement cancels — 엔진 메커니즘은 완비(signal + in-flight 렌더 취소). 다만 main.ts scanFigures가 아직 AbortController로 signal을 전달·abort하지 않아, 현재는 세대 가드로 결과만 버리고 스캔은 끝까지 진행됩니다("in-flight scans continue"). wiring이 남은 실작업.
• Blob/Object URL revoke — feat: add figure reference links, panel jumps, and crop flow #17 프리뷰가 dataURL 캐시(setDocument clear)라 revoke 대상이 없음. 현행 유지로 충족 검토 중.
• Memory QA 문서화 — figure 많은 장문 PDF로 스캔 전/후/문서 교체 후 상주 측정 예정.

→ 남은 4개는 Margin 측 wiring이라 열어 둡니다. #17 머지 후 후속 PR로 마무리하겠습니다.