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.Cpu.cs

@@ -18,7 +18,10 @@ public partial class MeshPainter
                     filterMode = FilterMode.Bilinear
                 },
                 pixels = SeedCpuPixels(mi, canonical),
-                painted = new bool[textureSize * textureSize],
+                // Seed the dilation-source marker with the UV islands so seeded (loaded /
+                // switched-in) content is protected from edge padding right away; the gutter
+                // stays paddable. Clone: painted[] is mutated per stroke.
+                painted = (bool[])GetIslandMask().Clone(),
                 fillStamp = new int[textureSize * textureSize],
                 dilateStamp = 0
             };