Matches the wall / slab / curve-wall tool pattern: emit
sfx:grid-snap only when the snapped position changes (cursor crosses
a grid cell), not every frame of mouse movement within the same cell.
Tracked via a `previousSnapRef` per tool, reset when the tool
re-activates.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
d17e083c77