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/Scenes/SampleScene.unity

@@ -2036,8 +2036,8 @@ MonoBehaviour:
   emissionIntensity: 20
   brushSize: 50
   brushMasks:
-  - {fileID: 10912, guid: 0000000000000000f000000000000000, type: 0}
-  - {fileID: 10300, guid: 0000000000000000f000000000000000, type: 0}
+  - {fileID: 2800000, guid: d07c01c40cc7be54da0ff3ae52484cd2, type: 3}
+  - {fileID: 2800000, guid: ccc205272b051834eb39593f647c7bf1, type: 3}
   activeBrushIndex: 0
   brushAlignment: 0
   brushRotation: 0