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/Scenes/SampleScene.unity

@@ -2038,6 +2038,10 @@ MonoBehaviour:
   brushMasks:
   - {fileID: 2800000, guid: d07c01c40cc7be54da0ff3ae52484cd2, type: 3}
   - {fileID: 2800000, guid: ccc205272b051834eb39593f647c7bf1, type: 3}
+  - {fileID: 2800000, guid: 04bc55e2853356e4f999b995d0066b33, type: 3}
+  - {fileID: 2800000, guid: 25780adbe09da488ba079bc33c4bbf0f, type: 3}
+  - {fileID: 2800000, guid: 25780adbe09da488ba079bc33c4bbf0f, type: 3}
+  - {fileID: 2800000, guid: 25780adbe09da488ba079bc33c4bbf0f, type: 3}
   activeBrushIndex: 0
   brushAlignment: 0
   brushRotation: 0
@@ -3354,13 +3358,13 @@ CharacterController:
   m_ProvidesContacts: 0
   m_Enabled: 1
   serializedVersion: 3
-  m_Height: 1
+  m_Height: 0.95
   m_Radius: 0.1
   m_SlopeLimit: 59
-  m_StepOffset: 0.3
+  m_StepOffset: 0.15
   m_SkinWidth: 0.01
   m_MinMoveDistance: 0.001
-  m_Center: {x: 0, y: 0.48, z: 0}
+  m_Center: {x: 0, y: 0.49, z: 0}
 --- !u!114 &1141039642
 MonoBehaviour:
   m_ObjectHideFlags: 0
@@ -3401,7 +3405,7 @@ MonoBehaviour:
   wallOffsetFollow: 12
   wallExitGrace: 0.15
   ceilingGrabDistance: 0.3
-  wallTopOutBoost: 0.5
+  wallTopOutBoost: 0.3
   cameraOrbit: {fileID: 0}
 --- !u!1 &1238479929
 GameObject: