Split mesh painter into partials and add occlusion via combined mask map

- Split MeshPainter into partial classes (Setup/Backend/Gpu/Cpu) for maintainability
- Add ambient occlusion painting, packed with metallic/smoothness into one
  MSO mask map (R=metallic, G=occlusion, A=smoothness) bound to both
  _MetallicGlossMap and _OcclusionMap, kept independently paintable via
  per-channel ColorMask
- Gate painting from a single IsChannelActive source so AO-only strokes work
- Blend albedo/emission in linear space on CPU to match the GPU framebuffer
- Pre-compensate data-channel writes for sRGB->linear so GPU matches CPU
- Harden GPU shader fallback to reflect real backend state

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

Assets/Shaders/MeshPaintBrush.shader

@@ -20,6 +20,7 @@ Shader "Hidden/MeshPaintBrush"
     {
         _BrushColor ("Brush Color", Color) = (1,0,0,1)
         _MaskTex ("Brush Mask", 2D) = "white" {}
+        _ColorWriteMask ("Color Write Mask", Float) = 15
     }
     SubShader
     {
@@ -29,6 +30,11 @@ Shader "Hidden/MeshPaintBrush"
             ZTest Always
             ZWrite Off
             Cull Off
+            // Per-channel write mask: lets the shared mask map (R=metallic, G=smoothness,
+            // B=occlusion) be painted by metallic/smoothness and occlusion independently.
+            // ColorWriteMask bits: R=8, G=4, B=2, A=1 (15 = all). Alpha (coverage) is always
+            // written so the dilation pass still sees the freshly painted texels.
+            ColorMask [_ColorWriteMask]
             // Source-over for color; alpha accumulates coverage toward 1 (painted marker
             // for the dilation pass). Soft edges keep partial alpha.
             Blend SrcAlpha OneMinusSrcAlpha, One OneMinusSrcAlpha

Assets/Shaders/MeshPaintPackMS.shader

@@ -1,14 +1,16 @@
-// Packs the metallic-smoothness WORK layout into the URP _MetallicGlossMap layout.
+// Packs the mask-map WORK layout into the URP mask-map layout.
 //
-// While painting, the metallic-smoothness map is kept in a "work" RenderTexture as
-//   R = metallic, G = smoothness, A = coverage (painted marker for the dilation pass).
+// While painting, the mask map is kept in a "work" RenderTexture as
+//   R = metallic, G = smoothness, B = occlusion, A = coverage (painted marker for dilation).
 // This lets the same brush + dilate shaders (which use alpha as the coverage marker)
-// drive it exactly like the albedo map. But URP's _MetallicGlossMap expects
-//   R = metallic, A = smoothness.
-// This pass converts work -> final after each stroke: out = (work.r, 0, 0, work.g).
+// drive it exactly like the albedo map. The packed result is one texture bound to BOTH
+//   _MetallicGlossMap (URP reads R = metallic, A = smoothness) and
+//   _OcclusionMap     (URP reads G = occlusion).
+// This pass converts work -> final after each stroke: out = (work.r, work.b, 0, work.g)
+//   = (metallic, occlusion, 0, smoothness).
 //
-// Both work and final are linear RenderTextures (metallic/smoothness are data, not
-// color), so no gamma conversion happens.
+// Both work and final are linear RenderTextures (these are data, not color), so no gamma
+// conversion happens.
 Shader "Hidden/MeshPaintPackMS"
 {
     Properties
@@ -35,7 +37,8 @@ Shader "Hidden/MeshPaintPackMS"
             fixed4 frag (v2f_img i) : SV_Target
             {
                 fixed4 c = tex2D(_MainTex, i.uv);
-                return fixed4(c.r, 0.0, 0.0, c.g); // R = metallic, A = smoothness
+                // work (R=metallic, G=smoothness, B=occlusion) -> (metallic, occlusion, 0, smoothness)
+                return fixed4(c.r, c.b, 0.0, c.g);
             }
             ENDCG
         }