fix(extensions): address review on hot-reload watcher

Per @codeant-ai's and @bito's review on #40084:

1. First-edit dropped on startup. Pre-populate baseline hashes from
   existing files in watched `dist` dirs via `prime_baseline()`, called
   once at watcher startup. Real edits now diff against the on-disk
   baseline instead of being silently swallowed as "first observation".

2. Move events used src_path. Atomic-build workflows (webpack tmp +
   rename into `dist`) mean `src_path` may point outside the watched
   tree. Use `dest_path` for `FileMovedEvent`, falling back to
   `src_path`.

3. Moves trigger regardless of content match. A move into/out of `dist`
   is itself the signal — don't gate it on hashing the (potentially
   missing) source.

4. Leading-edge debounce replaced with trailing debounce via
   `threading.Timer`. Each event resets the timer so the reload fires
   once after the build settles, instead of triggering immediately and
   dropping the writes that finish the build.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>