Drive parallax height and relief from the painted occlusion channel

Reuse the existing occlusion paint channel (no new map, MAP_COUNT stays 3):
0.5 is neutral, above 0.5 raises a hill, below 0.5 sinks a hole and also
fades ambient occlusion (saturate(2*occ)). Bind the MSO map to _ParallaxMap,
enable _PARALLAXMAP, derive a relief normal from the height gradient, and
reseed unpainted occlusion to 0.5 so untouched surfaces stay visually neutral.
Pin the _PARALLAXMAP runtime variant in the build keep-alive material.

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

Assets/Resources/PaintVariantKeepAlive.mat

@@ -28,6 +28,7 @@ Material:
   - _EMISSION
   - _METALLICSPECGLOSSMAP
   - _OCCLUSIONMAP
+  - _PARALLAXMAP
   m_InvalidKeywords: []
   m_LightmapFlags: 1
   m_EnableInstancingVariants: 0
@@ -78,7 +79,7 @@ Material:
         m_Scale: {x: 1, y: 1}
         m_Offset: {x: 0, y: 0}
     - _ParallaxMap:
-        m_Texture: {fileID: 0}
+        m_Texture: {fileID: 2800000, guid: c24e49ee8237bac42930ef868a325c48, type: 2}
         m_Scale: {x: 1, y: 1}
         m_Offset: {x: 0, y: 0}
     - _SpecGlossMap: