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.


https://github.com/user-attachments/assets/06d5ae7b-cc54-4cc5-a8b0-1cc30177cadb


## 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 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`** — removes a file's entries from the
store hashes
- **`RDoc::Store#clear_file_contributions`** — surgically removes a
file's methods, constants, comments, includes, extends, and aliases from
its classes/modules, preserving classes that span multiple files.
Supports `keep_position:` for server re-parse to preserve comment
ordering.
- **`RDoc::RDoc#relative_path_for`** — extracted path normalization
(against `options.root` and `options.page_dir`) shared by `parse_file`
and the server
- **`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

- **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).

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Co-authored-by: Sutou Kouhei <kou@clear-code.com>