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.

combat: NewEnemyShip layout

PH2 of combat task: enemy ship layout.

- ship.go: NewEnemyShip() returns same 5 rooms as NewPlayerShip
  with each GridX shifted +24 tiles. Pure data; no Ebitengine (IV1).
- ship_test.go: 2 tests covering room count and per-room shift
  (GridX +24, all other fields preserved).

Refs: docs/tasks/combat.md PH2

scaffold game loop and placeholder ship render