feat(mcpb): add read-only desktop extensions

[AlekseiSeleznev]

What changed

• add read-only MCPB packaging for Windows x64, macOS x64, and macOS arm64
• expose only search_docs, list_libraries, and find_version
• open the SQLite index in physical read-only mode
• add native CI builds and packaged-server smoke tests

Validation

• npm run typecheck
• npm run lint
• npm run build
• npm run build:mcpb:server
• npm test
• bundle scan excludes fetch_url, Chromium, Playwright, Xberg, and scraper pipeline code