fix(search): include generated collections

[mikemikimike]

Summary

Search currently keeps one collection per repository, so generated API declarations and changelog collections disappear whenever documentation from the same repository is selected.

What changed

• Select one current-or-default version per searchable artefact instead of per repository.
• Treat documentation, each generated declaration, and the global changelog as separate artefacts.
• Preserve the current artefact as the first result, while selecting the best available locale for other artefacts without duplicate fallback hits.
• Keep the existing lazy searchable-source initialization and exact/fuzzy search behavior unchanged.
• Add regression coverage for generated collections, per-artefact versions, and locale fallback.

Verification

• D:\CI\run-pre-pr.ps1 — PASS (prepr-20260910T015602Z-2011, Node 24)
• corepack pnpm check — lint, format, typecheck, tests (3108/3108), build, accessibility, and SEO checks passed

Closes #22

[TitusKirch]

Thanks for the PR! Issue #22 has already been resolved and closed via #42, so I’ll close this PR. Appreciate you taking the time to work on it.