Rebuilt CLI watcher around new fswatch package

[johnfav03]

The CLI watcher (tsgo --watch) was previously implemented with a polling-based approach, using a Run() loop - the key disadvantage of this system is that I would still call Stat() on tracked files even when they didn't change, which yielded an idle CPU usage of ~5% in the vscode repo.

With the addition of the fswatch package (PR #3980), we can rebuild the watcher around OS-level filesystem events, which results in an idle CPU usage of <1% on average in the vscode repo.

⚠️ Note: this branch is based on the fswatch branch, so only commits from 8230a72 onward contain relevant changes.

[Copilot]

Pull request overview

This pull request rebuilds tsgo --watch around the new internal/fswatch package (OS filesystem events + debouncing) instead of the previous polling loop, while keeping internal/vfs/vfswatch as a baseline-snapshot change detector for tracked compiler inputs.

Changes:

• Replaces the CLI watch loop with an event-driven fswatch subscription (recursive + ignore filter) and coalesced DoCycle signaling.
• Simplifies internal/vfs/vfswatch by removing polling/timing/debounce machinery, leaving only snapshot + compare logic.
• Introduces a new multi-backend internal/fswatch implementation (fanotify/inotify/kqueue/FSEvents/ReadDirectoryChangesW), plus extensive tests and documentation.

Reviewed changes

Copilot reviewed 38 out of 38 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/vfs/vfswatch/vfswatch.go	Removes polling-based watcher loop; keeps snapshot+compare watch-state logic.
internal/vfs/vfswatch/vfswatch_test.go	Updates tests for the new NewFileWatcher(fs) constructor.
internal/vfs/vfswatch/vfswatch_race_test.go	Updates race/fuzz tests to remove polling-interval related coverage.
internal/fswatch/windows.go	Adds Windows backend based on ReadDirectoryChangesW with overlapped I/O.
internal/fswatch/watcher.go	Adds core fswatch API surface, watcher lifecycle, per-dir state, and callback filtering.
internal/fswatch/walkdir.go	Adds generic walkDir implementation (portable baseline).
internal/fswatch/walkdir_windows.go	Adds Windows-optimized walkDir using FindFirstFile/FindNextFile.
internal/fswatch/walkdir_unix.go	Adds Unix walkDir using getdents/getdirentries for fewer stats.
internal/fswatch/walkdir_test.go	Adds tests validating walkDir behavior (symlinks, perms, entries, callbacks).
internal/fswatch/walkdir_other.go	Routes unsupported OSes to the generic walkDir implementation.
internal/fswatch/walkdir_dirent_noreclen.go	Adds DragonFlyBSD dirent helpers for reclen/inode extraction.
internal/fswatch/walkdir_dirent_linux.go	Adds Linux dirent helpers for reclen/inode extraction.
internal/fswatch/walkdir_dirent_fileno.go	Adds FreeBSD/OpenBSD/NetBSD dirent helpers for reclen/inode extraction.
internal/fswatch/walkdir_dirent_darwin.go	Adds macOS dirent helpers for reclen/inode extraction.
internal/fswatch/testutil_test.go	Adds retry-capable test harness to reduce platform flake impact.
internal/fswatch/README.md	Documents fswatch usage, watcher selection, and error semantics.
internal/fswatch/LICENSE	Adds upstream MIT license attribution.
internal/fswatch/kqueue.go	Adds kqueue backend for BSD/macOS (fd-per-entry model).
internal/fswatch/inotify_linux.go	Adds inotify backend for Linux.
internal/fswatch/fanotify_linux.go	Adds fanotify backend for Linux with rename probing/fallback.
internal/fswatch/fanotify_linux_test.go	Adds Linux fanotify-specific tests (availability, parsing, shutdown, cross-watcher).
internal/fswatch/fsevents_darwin.go	Adds macOS FSEvents backend (event classification and lifecycle).
internal/fswatch/fsevents_darwin_ffi.go	Adds cgo-free CoreFoundation/CoreServices/libdispatch FFI plumbing and NFC normalization helpers.
internal/fswatch/fsevents_darwin_ffi.s	Adds shared assembly trampolines and exported trampoline addresses.
internal/fswatch/fsevents_darwin_ffi_arm64.s	Adds arm64 FSEvents Create trampoline + callback shim assembly.
internal/fswatch/fsevents_darwin_ffi_amd64.s	Adds amd64 FSEvents Create trampoline + callback shim assembly.
internal/fswatch/fsevents_darwin_ffi_arm64_test.go	Adds static disassembly-based test ensuring the arm64 callback only touches safe registers.
internal/fswatch/fsevents_darwin_nfd_test.go	Adds macOS Unicode normalization regression tests (NFD/NFC mismatch scenarios).
internal/fswatch/event.go	Adds event kinds and per-path coalescing state machine.
internal/fswatch/eventlist_test.go	Adds unit tests for event coalescing and atomic drain semantics.
internal/fswatch/debounce.go	Adds per-backend debounce mechanism (min/max coalescing windows).
internal/fswatch/CHANGES.md	Documents divergence from upstream @parcel/watcher and lists bugfixes/features.
internal/fswatch/cgmanifest.json	Adds component governance manifest pointing at upstream source commit/tag.
internal/fswatch/canonicalize_other.go	Adds no-op path canonicalization for non-darwin platforms.
internal/fswatch/canonicalize_darwin.go	Adds NFC canonicalization for macOS path consistency.
internal/execute/watcher.go	Rebuilds CLI watcher to subscribe to fswatch events and signal DoCycle asynchronously.
.golangci.yml	Updates linter exclusion pattern to include internal/fswatch.