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/fixtures/manifest.json

@@ -4,7 +4,7 @@
     "threshold": null,
     "seed": null,
     "pxPerMm": 1,
-    "expected": { "widthMm": 1714, "heightMm": 675 },
-    "toleranceMm": 60
+    "expected": { "widthMm": 1712, "heightMm": 675, "holes": 0 },
+    "toleranceMm": 20
   }
 ]