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/PaintHud.Persistence.cs

@@ -68,6 +68,7 @@ public partial class PaintHud
         public Color emissionColor;
         public float emissionIntensity;
         public int brushSize;
+        public int activeBrushIndex;
         public int sprayDots;
         public float sprayDotSizeMin, sprayDotSizeMax, sprayDotAlphaMin, sprayDotAlphaMax;
     }
@@ -116,6 +117,7 @@ public partial class PaintHud
                 v.emissionColor = painter.emissionColor;
                 v.emissionIntensity = painter.emissionIntensity;
                 v.brushSize = painter.brushSize;
+                v.activeBrushIndex = painter.activeBrushIndex;
                 v.sprayDots = painter.sprayDots;
                 v.sprayDotSizeMin = painter.sprayDotSizeMin;
                 v.sprayDotSizeMax = painter.sprayDotSizeMax;
@@ -161,6 +163,7 @@ public partial class PaintHud
             painter.emissionColor = v.emissionColor;
             painter.emissionIntensity = v.emissionIntensity;
             painter.brushSize = v.brushSize;
+            painter.activeBrushIndex = v.activeBrushIndex;
             painter.sprayDots = v.sprayDots;
             painter.sprayDotSizeMin = v.sprayDotSizeMin;
             painter.sprayDotSizeMax = v.sprayDotSizeMax;