feat(docs): enhance search with TF-IDF relevance ranking (#40)

[diberry]

What this improves

The docs site has Pagefind (⌘K) for full-text keyword search. This PR enhances that existing search with an optional TF-IDF relevance ranking toggle — no second search, no separate overlay.

	Toggle OFF (default)	Toggle ON
Ranking	Pagefind keyword frequency	TF-IDF cosine similarity with title/heading boost
Best for	"Find pages containing this exact word"	"Which page best answers my question?"
Result quality	All pages with the term (flat list)	Re-ranked by relevance — best match first

Example: Searching "model pinning" with Pagefind returns every page mentioning "model". With relevance ranking ON, the Model Selection page ranks #1 because "pinning" appears in its title and headings (1.3x boost).

How it works

1. Open search with ⌘K / Ctrl+K (same as today)
2. Toggle "✨ Relevance ranking" checkbox (persists in localStorage)
3. When ON: Pagefind finds candidate results → TF-IDF re-ranks them by semantic relevance
4. When OFF: search works exactly as before

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Visual Change

The existing ⌘K search dialog now includes a "✨ Relevance ranking" checkbox below the search input:

• Unchecked (default): Standard Pagefind keyword search — unchanged behavior
• Checked: Results re-ranked by TF-IDF cosine similarity — best match surfaces first
• Toggle state persists in localStorage

📸 Screenshot — Search with Relevance Toggle

The ⌘K search dialog showing the "✨ Relevance ranking" checkbox and search results.

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Summary

Adds an optional relevance ranking enhancement to the existing Pagefind search:

1. Indexes ~108 markdown files into ~1,638 chunks at build time (build-search-index.mjs)
2. Re-ranks Pagefind results using TF-IDF with cosine similarity, boosting title (1.3x) and heading (1.15x) matches
3. Toggle is a checkbox in the search modal — off by default, persists in localStorage

No external APIs, no database, no runtime dependencies — everything runs in the browser.

Architecture (single search, not two)

• Removed SemanticSearch.astro as a separate component
• Removed floating FAB button and / key shortcut
• Removed separate search overlay
• Merged TF-IDF scoring logic into Search.astro as a re-ranking function
• Kept build-search-index.mjs — still generates the TF-IDF index at build time

Files Changed

File	Change
docs/scripts/build-search-index.mjs	New — Build-time chunker (108 files → 1,638 segments)
docs/src/components/Search.astro	Enhanced — Added TF-IDF re-ranking toggle + relevance scoring engine
docs/src/components/SemanticSearch.astro	Deleted — Merged into Search.astro
docs/src/layouts/DocsLayout.astro	Removed SemanticSearch import
test/docs-search.test.ts	New — 9 search quality tests

Testing

• ✅ 9/9 search quality tests pass (npx vitest run test/docs-search.test.ts)
• ✅ Docs build succeeds (cd docs && npm run build)

Decisions

1. Single search UX — Feature flag checkbox instead of a second search overlay
2. Off by default — Users opt in to relevance ranking
3. localStorage persistence — Toggle state remembered across sessions

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Relates to #40

[diberry]

Closing: upstream PR exists on bradygaster/squad. Work continues there.