node:fs — implement fs.watch / watchFile / promises.watch (plus cp, readlink, truncate, utimes) #28

Summary

node:fs in v0.10.5 covers sync, callback and promise APIs (readFile, createReadStream, realpath, symlink, chmod, mkdtemp, opendir, lstat, …) but fs.watch, fs.watchFile, fs.unwatchFile and fs.promises.watch are absent (crates/slice-engine/src/preludes/node_builtins.rs, verified at a0c723e).

Why it matters

File watching is the entry point for dev servers, hot-reload loops, log tailers and config reloaders. Many npm packages (chokidar, nodemon, vite, tsx --watch, pino rotation plugins) call fs.watch directly and currently throw TypeError: fs.watch is not a function.

Proposal

• Implement fs.watch(path, [options], listener) returning an FSWatcher (EventEmitter with change/rename/error/close, .close(), .ref()/.unref()).
• Implement fs.watchFile/unwatchFile (polling, Stats objects).
• Implement fs.promises.watch() as an async iterator.
• Back it with the notify crate already used by slice run --watch, and route events through runtime/event_loop.rs.
• Gate watched paths with the existing FsCap read allow-list; a watcher on a non-permitted path should fail with the same FsReadDenied error as readFile.
• VFS semantics: watchers should fire for in-memory VFS writes as well as host-disk writes, otherwise slice test time-travel rollbacks produce silent inconsistencies.

Also missing in node:fs

fs.cp, fs.readlink, fs.truncate, fs.utimes (+ sync/promises variants). These are smaller and could ship in the same change.