feat: trim textured background (wood grain) touching a smooth piece

Keep the piece's smooth core, then grow it back to restore the true edges.
Removes the wood-knot and grain leaks on the MDF-on-floor photo. Pieces that
are textured themselves are left untouched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/app-state.svelte.ts

@@ -1,4 +1,4 @@
-import { detectPrepared, prepareImage, preparedThreshold, type PreparedImage } from './detect';
+import { detectPrepared, disposePrepared, prepareImage, preparedThreshold, type PreparedImage } from './detect';
 import { prepareForExport } from './geometry';
 import { loadPhoto } from './load-image';
 import { shapeFromPolygons } from './margin';
@@ -69,7 +69,7 @@ export class AppState {
   }
 
   private prepare() {
-    this.prepared?.gray.delete();
+    if (this.prepared) disposePrepared(this.prepared);
     const { cv, image } = this;
     this.prepared = cv && image ? prepareImage(cv, image) : null;
     this.threshold = cv && this.prepared ? preparedThreshold(cv, this.prepared) : DEFAULT_THRESHOLD;