Add spray brush, paint interval, and per-channel paint opacity

Spray brush mode: bakes a procedural scattered-dot mask per stamp (random
position, size, alpha within the brush disc) and paints it through the
existing single-stamp pipeline; dot centers clamped to (0.5 - dotR) so the
spray footprint matches the solid brush radius.

Paint interval: float on MeshPainter; 0 keeps the continuous interpolated
stroke, >0 emits a single timed dab every x.xx seconds.

Color picker alpha: horizontal alpha bar on both HSV pickers, runtime-built
in Rebind (no prefab re-bake), passed through to brushColor.a / emissionColor.a.

Metallic/smoothness + occlusion paint opacity: new opacity fields applied in
both CPU and GPU backends (GPU MSO draw split into MS and occlusion sub-stamps
for independent opacity), with cloned alpha sliders wired into the HUD.

Also includes the prefab layout tweaks and scene adjustments.

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

Assets/Scenes/SampleScene.unity

@@ -446,6 +446,12 @@ MonoBehaviour:
   brushShader: {fileID: 0}
   dilateShader: {fileID: 0}
   packMsShader: {fileID: 0}
+  spray: 0
+  sprayDots: 24
+  sprayDotSizeMin: 0.05
+  sprayDotSizeMax: 0.18
+  sprayDotAlphaMin: 0.25
+  sprayDotAlphaMax: 1
   useGpu: 1
   paintAlbedo: 1
   paintMetallicSmoothness: 0
@@ -465,6 +471,7 @@ MonoBehaviour:
   brushAlignment: 0
   brushRotation: 0
   paintOnMoveOnly: 1
+  paintInterval: 0
   previewMode: 2
   previewOpacity: 1
   textureSize: 1024
@@ -1412,6 +1419,12 @@ MonoBehaviour:
   brushShader: {fileID: 0}
   dilateShader: {fileID: 0}
   packMsShader: {fileID: 0}
+  spray: 0
+  sprayDots: 24
+  sprayDotSizeMin: 0.05
+  sprayDotSizeMax: 0.18
+  sprayDotAlphaMin: 0.25
+  sprayDotAlphaMax: 1
   useGpu: 1
   paintAlbedo: 1
   paintMetallicSmoothness: 0
@@ -1431,6 +1444,7 @@ MonoBehaviour:
   brushAlignment: 0
   brushRotation: 0
   paintOnMoveOnly: 1
+  paintInterval: 0
   previewMode: 2
   previewOpacity: 1
   textureSize: 1024
@@ -1605,9 +1619,9 @@ Transform:
   m_GameObject: {fileID: 721670969}
   serializedVersion: 2
   m_LocalRotation: {x: -0, y: -0, z: -0, w: 1}
-  m_LocalPosition: {x: 0, y: 0, z: 0}
-  m_LocalScale: {x: 10, y: 10, z: 10}
-  m_ConstrainProportionsScale: 0
+  m_LocalPosition: {x: 0, y: 0, z: 11.5}
+  m_LocalScale: {x: 5, y: 5, z: 5}
+  m_ConstrainProportionsScale: 1
   m_Children: []
   m_Father: {fileID: 686187287}
   m_LocalEulerAnglesHint: {x: 0, y: 0, z: 0}
@@ -2756,6 +2770,12 @@ MonoBehaviour:
   brushShader: {fileID: 0}
   dilateShader: {fileID: 0}
   packMsShader: {fileID: 0}
+  spray: 0
+  sprayDots: 24
+  sprayDotSizeMin: 0.05
+  sprayDotSizeMax: 0.18
+  sprayDotAlphaMin: 0.25
+  sprayDotAlphaMax: 1
   useGpu: 1
   paintAlbedo: 1
   paintMetallicSmoothness: 0
@@ -2775,6 +2795,7 @@ MonoBehaviour:
   brushAlignment: 0
   brushRotation: 0
   paintOnMoveOnly: 1
+  paintInterval: 0
   previewMode: 2
   previewOpacity: 1
   textureSize: 1024