fix: Fall back to polling on macOS (#13622)
## Summary
- Preserve file-watcher startup failures for current and future
subscribers.
- Fail turbo watch immediately with the concrete startup error.
- Fall back from FSEvents to content-aware polling during macOS startup
only.
- Retain native watcher behavior on Linux and Windows.
Fixes #13607.
## Testing
- cargo test -p turborepo-filewatch --lib
- cargo test -p turborepo-lib --lib
- cargo check -p turborepo-filewatch --tests --target
aarch64-apple-darwin
- cargo clippy -p turborepo-filewatch --all-targets -- -D warnings
- cargo clippy -p turborepo-filewatch --tests --target
aarch64-apple-darwin -- -D warnings
- Pre-push format and TOML checks.
