arch: Redesign title browse page as expandable full-title TOC with inline content

[williamzujkowski]

Problem

The current navigation requires 3 clicks to read a statute section:

1. Browse → Title 18 (shows chapter list)
2. Title 18 → Chapter 7 (shows section list)
3. Chapter 7 → § 111 (shows content)

This makes it hard to get a holistic view of a title or quickly scan across chapters. Users wanting to read the entire code of a title must click into each chapter separately.

Proposal: Expandable Full-Title Table of Contents

Transform `/browse/title-{n}/` into an expandable, searchable TOC that shows the complete title structure:

```
Title 18 — Crimes and Criminal Procedure
1,399 sections across 126 chapters

[Search within Title 18...]

▸ Part I — Crimes (§§ 1-2725)
▾ Chapter 7 — Assault (9 sections)
§ 111 — Assaulting, resisting, or impeding... [⚡ changed in PL 117-159]
§ 112 — Protection of foreign officials...
§ 113 — Assaults within maritime and territorial jurisdiction
...
▸ Chapter 9 — Bankruptcy (4 sections)
▸ Chapter 10 — Biological Weapons (5 sections)
...
▸ Part II — Criminal Procedure (§§ 3001-3772)
...
```

Key features

1. Three-level hierarchy: Part → Chapter → Section (matching official USC structure)
2. Expand/collapse: Click chapter headings to show/hide section list
3. Section links: Click section to go to `/statute/{slug}/` detail page
4. Chapter links: Click chapter heading to go to `/browse/title-{n}/chapter-{n}/` (existing page)
5. Change indicators: Show which sections changed in recent release points (from diff manifest)
6. Status badges: Repealed/Reserved/Omitted/Transferred/Renumbered (inline, using frontmatter status field feat: Add 'status' frontmatter field to transformer output #90)
7. In-title search: Filter sections by keyword within the TOC
8. Section count per chapter: Show (N sections) badge

Information architecture impact

• `/browse/` — unchanged (title listing)
• `/browse/title-{n}/` — enhanced (full expandable TOC replaces simple chapter list)
• `/browse/title-{n}/chapter-{n}/` — kept (useful as direct chapter deep-link, also used by section page sidebar)
• `/statute/{slug}/` — unchanged (section detail)

The chapter page stays because:

• It's the target for chapter-level links from other sites
• Section page sidebar links to it
• Breadcrumbs reference it
• But it becomes optional — users can navigate Title→Section directly from the enhanced title page

Reference implementations

• Cornell LII (`law.cornell.edu/uscode/text/18`): Part → Chapter hierarchy with expandable sections
• OLRC (`uscode.house.gov`): JavaScript tree view with progressive expansion
• Both use expand/collapse; neither shows change tracking (our differentiator)

Technical approach

• Astro SSG: render full TOC at build time (all sections are available as content collection entries)
• Client-side: Svelte component for expand/collapse + search filter
• Change indicators: read from diff manifest at build time (which sections changed between recent tags)
• Performance: Only render section titles in TOC, not full content (avoids 60K section rendering)

Performance considerations

• Title 42 (largest) has ~7,500 sections across 163 chapters — TOC HTML is ~300KB
• Use `
  Details

  ` elements for zero-JS progressive enhancement
• Lazy-render expanded chapter sections for DOM performance
• Consider virtual scroll for very large titles

Files to modify

• `apps/web/src/pages/browse/[title].astro` — major rewrite to expandable TOC
• New: `apps/web/src/components/TitleToc.svelte` — interactive expand/collapse + search
• Possibly: `apps/web/src/pages/browse/[title].astro` needs access to all sections per title (currently only has chapters)

Depends on

• feat: Add 'status' frontmatter field to transformer output #90 (status frontmatter field) — merged ✅
• Epic: Historical US Code versions — import OLRC release points 2013-2026 #44 (historical import) — for change indicators

[williamzujkowski]

Progress: Phases 1-2 complete

Phase 1: Expandable TOC (PR #93) ✅

• Three-level hierarchy with `
  Details

  ` elements
• Expand/collapse all buttons
• Status badges, active/inactive counts
• Chapter links preserved as deep-link targets

Phase 2: In-title search filter (PR #94) ✅

• Debounced keyword filter on section titles
• Auto-expand/collapse chapters based on matches
• Match count display
• Only renders for titles with 20+ sections

Remaining Phases

• Phase 3: Per-section change indicators (enhance: Add per-section change indicators to title TOC #95) — show which PL changed each section
• Phase 4: Keyboard navigation within TOC (arrow keys to move between sections)
• Phase 5: URL hash support (expand specific chapter from URL like `#chapter-7`)

[williamzujkowski]

Phase 3 complete: Per-section change indicators (PR #97)

• Teal dot + PL label next to sections changed in tracked release points
• Data from new `changed-sections.json` index generated alongside diffs
• Key differentiator vs Cornell LII and OLRC

Phases complete

• Phase 1: Expandable TOC (feat: Expandable full-title table of contents for browse pages #93)
• Phase 2: In-title search filter (feat: In-title section search filter for expandable TOC #94)
• Phase 3: Change indicators (feat: Per-section change indicators in title TOC #97)
• Phase 4: Keyboard navigation
• Phase 5: URL hash support