Fix historical Reel Retina backfill and spotlight highlighting

[Happy (huangyangyu)]

Summary

• Preserve the canonical DPR1 poster PNG and historical poster HTML while generating quality-gated 2x/3x raster sources from the canonical PDF.
• Install Retina assets and rewritten HTML transactionally, and enable the fallback only for one uniquely identified historical layer inside the marked pixel host.
• Restore historical raster highlighting as a neutral 2px white spotlight with the surrounding poster faded; native HTML posters retain the existing DOM highlight path.
• Scope screenshot/pixel highlight QA exclusively to the Reel-owned historical-raster fallback. Native poster HTML is checked only for DOM highlight activation and cleanup; its durable pixel contract remains owned by paper2poster's exact warm/cold decoded-RGBA generation gate.
• Harden historical package QA for srcset/currentSrc, Retina rendering, proxy visibility and pixel effects, focus-core preservation, exact historical-proxy restoration, duplicate IDs, and failed-install recovery.

Retina sources are enabled only when PDF-derived output passes MAE <= 6, RMS <= 14, and maximum 32x32 tile RMS <= 30.

Validation

• Full public Daily Paper catalog repair: 55/55 unique Reels passed HTTP and file:// QA with 0 errors and 0 warnings.
• Public route reconciliation: 55/55 Reel pages and 55/55 nested posters returned HTTP 200; all 165 DPR1/2/3 raster URLs returned HTTP 200 with the expected PNG size.
• Every catalog entry passed all 8 artifact gates and exposed the neutral spotlight plus high-DPR image-rendering: auto runtime, with a unique scoped historical layer and canonical 1x src plus 1x/2x/3x srcset.
• Canonical DPR1 PNG and PDF hashes remained unchanged for all 55 papers.
• Targeted production verification for 2608.10915 and 2608.12307: 0 errors, 0 warnings; public DPR1/2/3 and spotlight behavior verified.
• Real 2608.06867 HTTP/file QA at DPR1/2/3: 0 errors.
• Real 2608.10299 full HTTP/file QA: 0 errors, 0 warnings.
• Native regression 2608.15089: the former mouseleave gate rejected 11 changed pixels at maximum channel delta 1; after scoping native QA to DOM lifecycle, full HTTP/file QA passes with 0 errors and 0 warnings.
• Additional real native HTML posters 2608.08311 and 2608.09802: full HTTP/file QA passes with 0 errors and 0 warnings.
• Historical-raster 2608.10915: final full HTTP/file QA still passes with 0 errors and 0 warnings after the native-scope correction.
• Negative coverage passed for localized corruption, wrong/missing PDF, oversized images, partial generation, transaction rollback, duplicate IDs, and full-focus washout.
• py_compile, both CLI help smoke tests, and git diff --check passed.
• Fresh end-to-end ResearchStudio-Reel rebuild of native 2608.15089 from the complete paper/poster/blog/video bundle: regenerated 10 section clips and 12 slide clips, validated on-demand delivery plus HTTP and file:// browser gates with 0 errors and 0 warnings, and preserved the canonical poster.html and poster.png SHA-256 hashes.

No test files, screenshots, reports, generated QA artifacts, or production bundles are included in this PR.