Add metallic, smoothness, and emission painting to mesh painter

Extend the painter from a single albedo map to three paint maps:
albedo (_BaseMap), packed metallic+smoothness (_MetallicGlossMap,
R=metallic A=smoothness), and emission (_EmissionMap/_EmissionColor).

- Per-channel toggles (paint albedo / metallic+smoothness / emission)
- Metallic/Smoothness sliders and emission color + intensity brush params
- One brush shader drives all maps; metallic-smoothness uses a work
  layout (R=metallic G=smoothness A=coverage) and a pack pass
  (Hidden/MeshPaintPackMS) to the URP layout after each stroke
- GPU and CPU backends mirror all three maps with the same mask and
  seam-dilation logic
- Material keywords (_METALLICSPECGLOSSMAP, _EMISSION) enabled and the
  new maps seeded from the original material values

Assets/Shaders/MeshPaintPackMS.shader

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+// Packs the metallic-smoothness WORK layout into the URP _MetallicGlossMap 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).
+// 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).
+//
+// Both work and final are linear RenderTextures (metallic/smoothness are data, not
+// color), so no gamma conversion happens.
+Shader "Hidden/MeshPaintPackMS"
+{
+    Properties
+    {
+        _MainTex ("Texture", 2D) = "white" {}
+    }
+    SubShader
+    {
+        Tags { "RenderType" = "Opaque" }
+        Pass
+        {
+            ZTest Always
+            ZWrite Off
+            Cull Off
+            Blend Off
+
+            CGPROGRAM
+            #pragma vertex vert_img
+            #pragma fragment frag
+            #include "UnityCG.cginc"
+
+            sampler2D _MainTex;
+
+            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
+            }
+            ENDCG
+        }
+    }
+    Fallback Off
+}

Assets/Shaders/MeshPaintPackMS.shader.meta

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