feat: add crop and canvas resize workflows

core/raster-layer.ts

@@ -1,7 +1,10 @@
 import type { BaseLayer } from "./base-layer";
 import type { AssetId } from "./id";
+import type { Rect } from "./geometry";
 
 export type RasterLayer = BaseLayer & {
   type: "raster";
   assetId: AssetId;
+  /** Non-destructive source pixels displayed by this layer. Absence means the full asset. */
+  sourceRect?: Rect;
 };