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>

tests/detect.test.ts

@@ -1,5 +1,5 @@
 import { beforeAll, describe, expect, it } from 'vitest';
-import { detect, detectPrepared, otsuThreshold, prepareImage, preparedThreshold } from '../src/lib/detect';
+import { detect, detectPrepared, disposePrepared, otsuThreshold, prepareImage, preparedThreshold } from '../src/lib/detect';
 import { bounds } from '../src/lib/geometry';
 import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
 import type { DetectParams } from '../src/lib/types';
@@ -126,7 +126,27 @@ describe('detect', () => {
         expect(reused.holes).toHaveLength(direct.holes.length);
       }
     } finally {
-      prepared.gray.delete();
+      disposePrepared(prepared);
     }
   });
+
+  it('cuts off textured background (wood grain) that touches a smooth piece', () => {
+    // Smooth dark piece x 150..449, y 100..299 on a light floor, with a dark grainy knot touching its top-left.
+    const knotted = grayImage(600, 450, (x, y) => {
+      if (inRect(x, y, 150, 100, 450, 300)) return 80;
+      if (inRect(x, y, 90, 50, 170, 110)) return Math.floor(x / 3) % 2 ? 40 : 110;
+      return 200;
+    });
+    const b = bounds(detect(cv, knotted, params({ threshold: 140 }))!.outer);
+    expect(b.minX).toBeGreaterThanOrEqual(144);
+    expect(b.minY).toBeGreaterThanOrEqual(94);
+    near(b.maxX, 449); near(b.maxY, 299);
+  });
+
+  it('keeps a piece that is itself textured (e.g. plywood grain)', () => {
+    const plywood = grayImage(600, 450, (x, y) =>
+      inRect(x, y, 150, 100, 450, 300) ? (Math.floor(x / 3) % 2 ? 40 : 110) : 200);
+    const b = bounds(detect(cv, plywood, params({ threshold: 140 }))!.outer);
+    near(b.minX, 150); near(b.maxX, 449); near(b.minY, 100); near(b.maxY, 299);
+  });
 });