## Summary

Add `dev: Debug Filesystem Watching`, which opens a singleton, app-wide
debug window for local filesystem watching.

- Capture native and polling notifications before filtering/coalescing,
including lost-sync/rescan markers and watcher initialization,
registration, and removal errors.
- Show Raw Events, Watch Roots, and Scan Exclusions in separate
read-only editors with normal selection and copying.
- Record only while the window is open, retain the latest 10,000 events,
and periodically compact editor buffers so deleted text/history does not
accumulate indefinitely.
- Display local timestamps and concise event names; export full raw
details, roots, exclusions, dropped-event counts, and Zed/OS version
information as JSON.
- Declare the renderer source that `gpui_apple`'s build script reads in
Corgi's sandbox configuration.

This is diagnostic tooling, not a change to watcher behavior. It
captures local watcher-level events; it does not yet trace worktree scan
start/completion or remote filesystems. Scan exclusions are labeled
separately from OS watch roots because excluded files can still generate
notifications.

## Related issues

These reports motivate the capture tooling; this PR does not claim to
fix them:

- [#63254: Project and Git panels stop updating when the macOS FSEvents
stream limit is
reached](https://github.com/zed-industries/zed/issues/63254)
- [#63174: Atomic file replacement leaves Linux buffers
stale](https://github.com/zed-industries/zed/issues/63174)
- [#58678: Worktree stops opening files/expanding folders after a
lost-sync rescan
storm](https://github.com/zed-industries/zed/issues/58678)
- [#53480: High CPU after watcher lost sync under low file-descriptor
limits](https://github.com/zed-industries/zed/issues/53480)

## Validation

- `corgi build -p zed`
- `corgi test -p fs --lib --features test-support fs_watcher` — 19 tests
passed
- `corgi test -p fs --features test-support --test integration
watcher_diagnostics` — 2 tests passed, including real OS notification
delivery
- `cargo test -p zed watcher_debug::tests` — 7 tests passed, covering
formatting, singleton lifetime, tab layout, copying/read-only behavior,
selection preservation, compaction and retained storage, JSON metadata,
cancellation, and save failures
- Rust formatting and `git diff --check`
- Manual UI smoke testing on macOS during development

The UI tests use Cargo because the installed Corgi version omits
`CARGO_BIN_NAME` when compiling the binary test target. Normal Corgi
builds pass.

## Reviewer note

The required README review acknowledgment is intentionally left for the
author to remove manually after review.

## Suggested .rules additions

For bounded diagnostic logs backed by an editor, trimming visible rows
does not bound memory: the text buffer retains deleted text and edit
history. Periodically replace the buffer/editor while preserving
selection and scroll state, and test retained storage rather than only
visible row count.

Release Notes:

- Added `dev: Debug Filesystem Watching` to inspect local watcher
events, watch roots, and scan exclusions, and export diagnostic captures
as JSON.