enhance: Add per-section change indicators to title TOC

[williamzujkowski]

Context

Phase 3 of #92 (IA epic). The expandable title TOC and in-title search are live. Next: show which sections changed in recent release points.

Proposed approach

At build time, generate a per-section change index from the diff data:

1. Read diff manifest pairs
2. For each pair, list the section files that have diffs
3. Write a JSON index: `{ "statutes/title-18/chapter-7/section-111.md": ["pl-117-159"] }`
4. Title page reads this index and shows change badges

Display

• Small lightning bolt or colored dot next to recently-changed sections
• Tooltip showing which PL changed it: "Changed in PL 117-159"
• This is the key differentiator vs Cornell LII and OLRC (neither shows change tracking)

Files

• `scripts/generate-diffs.ts` — add section-level change index output
• `apps/web/src/pages/browse/[title].astro` — read index and render indicators

Depends on

• Diff manifest data available at build time ✅
• Expandable TOC (feat: Expandable full-title table of contents for browse pages #93 merged) ✅