feat(occupancy): grid builder from ultrasonic frames

### Summary

### Tasks
- Create `packages/monad-occupancy/grid.py`
- `build_grid(frame, cell_size_m=0.1, range_m=2.0)` returns 2D list.
- Unit tests: empty, obstacle, range clip.
- Telemetry metric: occ_occupied_cells.
- No external deps.

### Acceptance
Tests green; README demo; telemetry logs occupied cell count.


### Area

occupancy

### Priority

high