Add server mode with live reload (rdoc --server)

[st0012]

A better attempt of #1151

Implement rdoc --server[=PORT] for previewing documentation with automatic browser refresh on source file changes. The server parses all sources on startup, serves pages from memory via the Aliki generator, and watches for file modifications, additions, and deletions — re-parsing only what changed.

rdoc-server-live-reload-demo.mp4

Changes

• New RDoc::Server (lib/rdoc/server.rb) — minimal HTTP server using Ruby's built-in TCPServer (no WEBrick or external dependencies)
  • Thread-per-connection with Connection: close
  • Persistent Aliki generator instance rendering pages to strings
  • In-memory page cache and search index cache with full invalidation on changes
  • Live reload via inline JS polling /__status endpoint every 1 second
  • Background file watcher polling source file mtimes every 1 second
  • Incremental re-parse: only changed files are re-parsed, old data removed first
  • Detection of new and deleted source files
• --server[=PORT] CLI option (default port 4000) and rdoc:server Rake task
• RDoc::Store#remove_file — recursively removes a file's classes, modules, and C variable maps from the store
• Darkfish#refresh_store_data — extracted for reuse by the server after re-parsing
• RDoc::Servlet → RDoc::RI::Servlet — moved to clarify RI-specific usage

Security & robustness

• Binds to 127.0.0.1 only (localhost)
• Path traversal protection in asset serving (File.expand_path containment check)
• Proper HTTP error responses: 400, 404, 405, 500
• 5-second IO.select read timeout on client sockets
• Mutex protects all store mutations, generator refresh, and cache invalidation atomically
• Individual parse_file errors rescued so one failure doesn't block remaining files
• Watcher thread uses @running flag with clean shutdown via Thread#join

Known limitations

• Reopened classes and file deletion: if a class is defined across multiple files, deleting one file removes the entire class from the store. Saving the remaining file triggers a re-parse that restores it.
• Full cache invalidation: any file change clears all cached pages. Rendering is fast (~ms per page); parsing is the expensive part and is done incrementally.
• Template/CSS changes: require server restart (only source files are watched).

[matzbot]

🚀 Preview deployment available at: https://9360f302.rdoc-6cd.pages.dev (commit: a3bd278)