feat: search improvements — pagination, heading scroll, and body text indexing

Problem

Several search limitations affect usability:

1. Hardcoded LIMIT 20 — no pagination or cursor support for large result sets
2. Heading matches don't scroll — clicking a heading result navigates to the page but doesn't scroll to the `#anchor`
3. Static index only indexes headings — `build-search-index.ts` extracts headings but not prose body text, making full-text search incomplete (the runtime FTS index in `search.ts` does index body text, but the two are inconsistent)
4. BM25 weights are undocumented magic numbers — `bm25(search_fts, 10.0, 5.0, 1.0)` with no explanation of tuning rationale

Suggested Improvements

• Add `offset`/`limit` query params to the search API
• After navigating to a heading result, scroll the anchor into view on the client
• Unify the two search indexing paths (static builder vs runtime builder)
• Document or make configurable the BM25 weight tuning

Start by reading build-search-index.ts and search.ts to compare the static and runtime indexing paths, then trace the search API and client navigation for the existing LIMIT 20 and heading-result behavior. Done means pagination parameters work, heading links scroll to their anchors, body text is indexed consistently, and the BM25 weights are documented or configurable.