Tilemap cells are stored compactly in the project file: each layer keeps a tile list, an origin and one run-length encoded string per row; older projects with a cell map still load (#134)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/model/tilemap.ts

@@ -6,6 +6,7 @@
  * Cells are sparse – a map from `"x,y"` to a tile id – rather than one entry per grid square.
  * A map is what painting, erasing and reading a neighbour all want, and changing the grid size then
  * moves nothing: what falls outside is simply not drawn and comes back when the grid grows again.
+ * The project file stores them more compactly (#134, `store/tilemapCells.ts`).
  */
 import type { TileAsset, TilemapLayer } from './Asset';
 import { type Neighbours, type TileQuadrants, tileQuadrants } from './autotile';