Wire local search via @easyops-cn/docusaurus-search-local (#193 of Fallout)

[ChrisonSimtian]

Summary

Closes ChrisonSimtian/Fallout#193 — adds in-bundle search to https://docs.fallout.build/. The index is generated at build time and shipped with the static assets; no external service involved.

Why this plugin / why this version

• Plugin: `@easyops-cn/docusaurus-search-local` is the standard self-contained search add-on for Docusaurus.
• Pinned to 0.54.1, not 0.55.x. The 0.55 line introduced `open-ask-ai` (an AI Q&A widget) as a hard peer dependency. We don't want that feature, and we don't want that extra runtime dependency on the docs site. 0.54.1 is the last release before that change, supports Docusaurus 3 + React 19, and is otherwise feature-complete for our needs. If we ever want the AI widget we can upgrade and install `open-ask-ai` deliberately.

Config choices

• `hashed: true` — content-hashed index filenames so CDN/Pages caching invalidates correctly when content changes.
• `indexBlog: false` — `blog: false` in the classic preset; without this the plugin warns.
• `docsDir: 'fallout-source/docs'` — markdown source lives in the sibling Fallout repo, checked out into `./fallout-source/` at build time (matches the existing `docs` preset `path: './fallout-source/docs'`). Without this, the plugin warns that the default `./docs` doesn't exist.
• `highlightSearchTermsOnTargetPage: true` + `explicitSearchResultPath: true` — small UX wins on the search results page and after landing on a hit.

Verification

• Local `npm run build` cannot complete on this machine due to a pre-existing Node 26 SSR issue affecting `main` already (see PR Fix broken-link warnings; enforce broken-link checks (#192 of Fallout) #2's notes). But the plugin loads cleanly — no config-time errors, and the previous `docsDir doesn't exist` warning is gone after pointing it at the right path.
• Real validation happens on CI here (Node 22) post-merge. If the build succeeds, the next deploy of docs.fallout.build will ship the search box in the navbar.

What this PR does NOT do

• No styling changes to the search box (default Docusaurus styling is fine).
• No keyboard-shortcut customisation (defaults to `/` to focus search).
• No tuning of result limits or snippet length.

If any of those want polish after we see the deployed UX, that's follow-up.

Refs

• Closes ChrisonSimtian/Fallout#193 (one of three follow-ups from the closed parent #41).
• Sibling PRs already merged: ChrisonSimtian/Fallout#194 (README link), ChrisonSimtian/Fallout#195 + Falloutdocs#2 (broken-link cleanup).