fix: fall back when installed Next.js has no bundled docs

[gaojude]

An installed Next.js 16.0.7 package has no dist/docs, but nextjs_docs returns use_bundled_docs and suggests reinstalling dependencies. Reinstalling the same version cannot recover files that release does not ship.

Check the installed package contents before returning bundled-doc instructions. An installed release without docs receives use_online_docs with a version-aware fallback; a declared but missing modern dependency receives install_required. Update the README and add a patch changeset.

Validation:

• The regression assertions failed against the original implementation.
• pnpm build, pnpm typecheck, and the complete pnpm exec vitest run: 34 tests passed.
• Real stdio MCP calls: installed Next.js 16.0.7 receives the online fallback, while 16.3.4 retains its bundled-doc response.

Local validation used macOS arm64, Node.js 24.19.0, and pnpm 9.15.9. The uploaded GitHub-signed commit has the same Git tree as the tested checkout.