Refactor STTextView layout system for improved viewport stability and accuracy

- Replace snapshot-based layout with deterministic relayout system
- Properly track last used text fragments to reduce flickering and memory use
- Clamp viewport bounds to avoid overscroll issues
- Improve content size estimation using FB15131180 workaround
- Refactor resizing system to match NSTextView's frame behavior
- Fix horizontal/vertical resize API to be more intuitive
- Add support for suppressing layout during live resize for performance
- Remove dependency on SnapshotTesting and outdated snapshot tests
- Add comprehensive SizeToFitTests to validate behavior parity with NSTextView
- Drop macOS deployment target to 14.0 and remove snapshot fixtures accordingly