Fix frozen rim pixels via GPU coverage stamp; add brush masks

Replace the CPU island-mask dilation guard on the GPU backend with a
coverage pass rendered through the same brush rasterizer, so sub-pixel
rim texels that the GPU brush can never reach fall into the gutter and
stay paintable (no more frozen loaded pixels at island edges).

Also includes user content: additional brush masks (dirt) with an
expanded mask selector in the HUD prefab, and CharacterController /
wall-climb inspector tuning.

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

Assets/Shaders/MeshPaintBrush.shader

@@ -103,6 +103,42 @@ Shader "Hidden/MeshPaintBrush"
             }
             ENDCG
         }
+
+        // Pass 1 - coverage: rasterizes the mesh in UV space (identical vertex transform to
+        // the brush) and writes alpha = 1 on exactly the texels the brush can paint. Used to
+        // seed the dilation guard (display RT alpha) so it matches the GPU rasterizer instead
+        // of a CPU point-in-triangle approximation. Texels the brush can never reach (sub-pixel
+        // island borders) stay alpha = 0 -> the gutter, so dilation can still overpaint them.
+        Pass
+        {
+            ZTest Always
+            ZWrite Off
+            Cull Off
+            Blend Off
+            ColorMask A
+
+            CGPROGRAM
+            #pragma vertex vert
+            #pragma fragment frag
+            #include "UnityCG.cginc"
+
+            struct appdata { float4 vertex : POSITION; float2 uv : TEXCOORD0; };
+            struct v2f { float4 pos : SV_POSITION; };
+
+            v2f vert (appdata v)
+            {
+                v2f o;
+                float2 p = v.uv * 2.0 - 1.0;
+                #if UNITY_UV_STARTS_AT_TOP
+                    p.y = -p.y;
+                #endif
+                o.pos = float4(p, 0.0, 1.0);
+                return o;
+            }
+
+            fixed4 frag (v2f i) : SV_Target { return fixed4(0, 0, 0, 1); }
+            ENDCG
+        }
     }
     Fallback Off
 }