feat(crew): restrict movement to room cell slots

Add per-room cell slots and limit crew movement to unoccupied cells.

- Add Cells field to Room and populate two slots per player room.
- Shift enemy-ship cells by the same x-offset as rooms.
- Build a cellSet lookup and gate moves on it, skipping occupied cells.
- Add cellOccupied check so two crew cannot share a slot.
- Render dim cell markers in render.go.
- Cover cellSet and occupancy with tests; tighten hit-flash test timing.

crew-movement: walkability + BFS pathfinding with tests