scroll: Lock trackpad wheel gestures to their starting axis (#2648)

Wireup https://github.com/zed-industries/zed/pull/61797.

- `ScrollableMask` — per-gesture axis lock (follow-up to #2608)
- `Scrollable`, `VirtualList` — no more cross-axis remapping (behavior
change)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>