Tilemaps are a new placeable asset type that is painted with autotiles in the scene view, with layers, brush, eraser and rectangle, and is drawn at runtime and in the export (#128)

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

src/core/commands/assetCommands.ts

@@ -41,6 +41,9 @@ export function buildAsset(type: AssetType, name: string, id = newId()): Asset {
       return { ...base, type: 'animation', root: '', tracks: [] };
     case 'autotile':
       return { ...base, type: 'autotile', pictures: [] };
+    // A tilemap without a layer could not be painted on, so it starts with one.
+    case 'tilemap':
+      return { ...base, type: 'tilemap', layers: [{ id: newId(), name: 'Layer 1', cells: {} }] };
     case 'statemachine':
       return { ...base, type: 'statemachine', graph: createEmptyGraph() };
     case 'shader':