Add HUD brush mask selector and fix dilation seam artifacts

- Add mask selector with previews to the brush card (data-driven from
  painter.brushMasks); persist activeBrushIndex with HUD layout.
- Fix alpha/coverage artifacts when overpainting after load or backend
  switch: derive the dilation source marker from a deterministic UV island
  mask instead of resetting it to "unpainted" on every seed.
- Import the two final brush mask textures (Brushes/).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/MeshPainter.cs

@@ -204,6 +204,7 @@ public partial class MeshPainter : MonoBehaviour
     private Vector3[] _triCenter;   // centroid per triangle (CPU culling)
     private float[] _triRadius;     // circumscribed radius per triangle (CPU culling)
     private float _avgUvToLocal = 1f; // average UV unit -> local unit
+    private bool[] _islandMask;       // texel lies inside a UV island: dilation source / overwrite guard (built once)
 
     private bool _hasLastScreen;
     private Vector2 _lastScreen;