FEAT add livetext output granularity

[Yinwhe]

1. Add LiveText flat output granularity parameter unit: 'token' (default) or 'line' (full line with bbox).
2. Fix Vision performRequests_error_ handling by unpacking (ok, error) to avoid treating a non-empty tuple as True.
3. Minor: wrap LiveText analysis in objc.autorelease_pool(); add font fallback in annotate_PIL.
4. Docs: add a short note about unit after the LiveText notice.

[straussmaximilian]

Thanks a lot!