fix: show furniture-occupied tiles in the floor plan editor

The editor never requested occupied tiles, so tiles holding furniture
were indistinguishable from empty floor and could be edited/voided.

- request GetOccupiedTilesMessageComposer when the editor opens
- handle RoomOccupiedTilesMessageEvent -> SET_OCCUPIED_TILES
- new Tile.occupied flag (kept separate from `blocked`/void): occupied
  tiles render with a distinct marker and are protected from PAINT/
  ERASE/ADJUST and brush-to-selection edits
- occupied is purely informational and never changes the saved tilemap
  (no accidental voiding of floor under furni)

Tests: reducer cases for SET_OCCUPIED_TILES + edit protection; container
test asserts the occupied event is non-destructive on save; route the
canvas pointer test through elementFromPoint (jsdom has no getScreenCTM).

src/components/floorplan-editor/views/FloorplanTile.tsx

@@ -104,6 +104,17 @@ const FloorplanTileImpl: FC<Props> = ({ row, col, tile, selected, isDoor, southH
                 stroke="#222"
                 strokeWidth={ 0.5 }
             />
+            { tile.occupied && (
+                <polygon
+                    data-testid="occupied-marker"
+                    points={ points }
+                    fill="rgba(249, 115, 22, 0.40)"
+                    stroke="#f97316"
+                    strokeWidth={ 1 }
+                    strokeDasharray="2 2"
+                    pointerEvents="none"
+                />
+            ) }
             { selected && (
                 <polygon
                     data-testid="selection-ring"