Per-symbol API data: shrink /docs SSR payload and restore API-page head()/notFound()

[dogmar]

Follow-up to #9 (merged as cdafc34). Two related regressions in the API-reference docs were deferred from that PR's review because the clean fix is a single design change rather than a patch.

Problem

1. The /docs layout loader ships the full API symbols JSON in every page's SSR payload.
website/src/routes/docs.tsx returns apiData (the whole ~250KB api-data/symbols.gen.json) from its loader. The dynamic import() keeps it out of the client JS bundle, but returning it from the loader serializes it into the dehydration payload of every /docs/* page — including pure-content pages like /docs/getting-started that render no API tables. This defeats the lazy-loading goal: pre-#9, the layout shipped only {name, kind} nav entries and symbol pages fetched a single symbol server-side.

2. API symbol pages lost head() and notFound() semantics.
website/src/routes/docs/api/$symbol.tsx:

• No head() — every API page inherits the generic site title and has no per-symbol <title>/meta description (SEO + tab-title regression vs. main).
• The loader never throws notFound(), so /docs/api/DoesNotExist SSRs with HTTP 200, the declared notFoundComponent is dead code, and it's byte-duplicated by an inline "Symbol not found" fallback in the component body.

Recommended approach (option C from the #9 discussion)

Split the build-time output into per-symbol chunks instead of one monolith:

• website/scripts/extract-api.ts emits api-data/<symbol>.gen.json per symbol, plus a small api-data/index.gen.json (name → kind) for nav + link detection.
• docs.tsx loader returns only the small index (drop apiData from loader data).
• docs/api/$symbol.tsx loader imports the single <symbol>.gen.json, throws notFound() when missing (server fn or loader), and sets head() title/description from it. Delete the inline fallback — let notFoundComponent handle it.
• TypeLink tooltips dynamic-import the hovered symbol's chunk on demand instead of reading a page-wide apiData.
• Broaden the ambient declaration added in docs: precompile Markdoc into React route components at build time #9 from */api-data/symbols.gen.json to */api-data/*.gen.json so all chunks resolve without being checked in (they stay gitignored + generated, same as today).

Also in scope (same area, smaller)

• Restore a branded docs 404. Deleting the dynamic $slug.tsx in docs: precompile Markdoc into React route components at build time #9 removed the docs-layout notFoundComponent, so unknown /docs/<slug> URLs now hit TanStack Router's bare default not-found instead of the styled page. Add a notFoundComponent to docs.tsx (or a root defaultNotFoundComponent).

Verification

• Confirm /docs/getting-started SSR HTML no longer contains the symbols payload (was ~250KB).
• /docs/api/<Known> has the correct <title> and meta description; /docs/api/<Bogus> returns 404 and renders the branded not-found.
• API tables still SSR (don't regress to client-only rendering).
• vp run ready passes with the file absent (ambient declaration covers the per-symbol chunks).

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