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.cs

@@ -63,6 +63,8 @@ public partial class PaintHud : MonoBehaviour
     private const string SliderSprayDotSizeMax = "Slider_SprayDotSizeMax";
     private const string SliderSprayDotAlphaMin = "Slider_SprayDotAlphaMin";
     private const string SliderSprayDotAlphaMax = "Slider_SprayDotAlphaMax";
+    private const string MaskSelector = "MaskSelector";
+    private const string MaskSwatchPrefix = "MaskSwatch_";
 
     // Panels made draggable at runtime - grabbable from any non-widget surface (both prefab and code-gen paths).
     private static readonly string[] DraggablePanels =
@@ -206,6 +208,8 @@ public partial class PaintHud : MonoBehaviour
 
         WireSlider(panel, SliderBrushSize, painter ? painter.brushSize : 16f, v => { if (painter) painter.brushSize = Mathf.RoundToInt(v); });
 
+        WireMaskSelector(panel);
+
         WireSlider(panel, SliderSprayDots, painter ? painter.sprayDots : 24f, v => { if (painter) painter.sprayDots = Mathf.RoundToInt(v); });
         WireSlider(panel, SliderSprayDotSizeMin, painter ? painter.sprayDotSizeMin : .05f, v => { if (painter) painter.sprayDotSizeMin = v; });
         WireSlider(panel, SliderSprayDotSizeMax, painter ? painter.sprayDotSizeMax : .18f, v => { if (painter) painter.sprayDotSizeMax = v; });
@@ -250,6 +254,8 @@ public partial class PaintHud : MonoBehaviour
         SetSlider(panel, SliderSprayDotSizeMax, painter.sprayDotSizeMax);
         SetSlider(panel, SliderSprayDotAlphaMin, painter.sprayDotAlphaMin);
         SetSlider(panel, SliderSprayDotAlphaMax, painter.sprayDotAlphaMax);
+
+        RefreshMaskHighlights();
     }
 
     private static void SetToggle(Transform panel, string name, bool value)
@@ -271,6 +277,82 @@ public partial class PaintHud : MonoBehaviour
         if (picker) picker.SetColor(color);
     }
 
+    // Active = accent frame, inactive = subtle frame. Selection lives on the painter (activeBrushIndex);
+    // the swatch click only sets it - the painter resolves the new mask on its next Update.
+    private static readonly Color MaskFrameSelected = new Color(0.30f, 0.62f, 1f, 1f);
+    private static readonly Color MaskFrameUnselected = new Color(0.18f, 0.18f, 0.21f, 1f);
+
+    /// <summary>
+    /// Binds the baked mask swatches (one per <c>MaskSwatch_i</c>) to the painter's brush masks:
+    /// each swatch previews <c>painter.brushMasks[i]</c> and selects it on click. A swatch with no
+    /// matching mask is hidden, so the prefab can carry more swatches than the painter has masks.
+    /// </summary>
+    private void WireMaskSelector(Transform panel)
+    {
+        var selector = FindDeep(panel, MaskSelector);
+        if (!selector) return;
+        var masks = painter ? painter.brushMasks : null;
+
+        for (int i = 0; ; i++)
+        {
+            var swatch = FindDeep(selector, MaskSwatchPrefix + i);
+            if (!swatch) break;
+
+            bool has = masks != null && i < masks.Count && masks[i];
+            swatch.gameObject.SetActive(has);
+            if (!has) continue;
+
+            var preview = swatch.Find("Preview");
+            var previewImg = preview ? preview.GetComponent<Image>() : null;
+            if (previewImg) previewImg.sprite = MaskSprite(masks[i]);
+
+            var btn = swatch.GetComponent<Button>();
+            if (btn)
+            {
+                int idx = i; // capture per swatch
+                btn.onClick.RemoveAllListeners();
+                btn.onClick.AddListener(() => SelectMask(idx));
+            }
+        }
+
+        RefreshMaskHighlights();
+    }
+
+    private void SelectMask(int index)
+    {
+        if (painter) painter.activeBrushIndex = index;
+        RefreshMaskHighlights();
+    }
+
+    private void RefreshMaskHighlights()
+    {
+        if (!_panel || !painter) return;
+        var selector = FindDeep(_panel.transform, MaskSelector);
+        if (!selector) return;
+
+        for (int i = 0; ; i++)
+        {
+            var swatch = FindDeep(selector, MaskSwatchPrefix + i);
+            if (!swatch) break;
+            var frame = swatch.GetComponent<Image>();
+            if (frame) frame.color = i == painter.activeBrushIndex ? MaskFrameSelected : MaskFrameUnselected;
+        }
+    }
+
+    // One sprite per mask texture, reused across rewires (display only - no CPU readback needed).
+    private static readonly System.Collections.Generic.Dictionary<Texture2D, Sprite> _maskSprites
+        = new System.Collections.Generic.Dictionary<Texture2D, Sprite>();
+
+    private static Sprite MaskSprite(Texture2D tex)
+    {
+        if (!tex) return null;
+        if (_maskSprites.TryGetValue(tex, out var cached) && cached) return cached;
+        var s = Sprite.Create(tex, new Rect(0f, 0f, tex.width, tex.height), new Vector2(0.5f, 0.5f), 100f);
+        s.hideFlags = HideFlags.HideAndDontSave;
+        _maskSprites[tex] = s;
+        return s;
+    }
+
     /// <summary>
     /// Attaches (idempotently) a <see cref="DraggablePanel"/> to the whole panel so it can be dragged
     /// from any of its non-interactive surface (header, padding, labels, background); the component