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/Scripts/PaintHud.cs

@@ -51,7 +51,9 @@ public class PaintHud : MonoBehaviour
     private const string PickerEmission = "Picker_Emission";
     private const string SliderMetallic = "Slider_Metallic";
     private const string SliderSmoothness = "Slider_Smoothness";
+    private const string SliderMetallicSmoothnessAlpha = "Slider_MetallicSmoothnessAlpha";
     private const string SliderOcclusion = "Slider_Occlusion";
+    private const string SliderOcclusionAlpha = "Slider_OcclusionAlpha";
     private const string SliderEmissionIntensity = "Slider_EmissionIntensity";
     private const string SliderBrushSize = "Slider_BrushSize";
 
@@ -168,14 +170,16 @@ public class PaintHud : MonoBehaviour
         WirePanelFade(panel, PanelEmission, ToggleEmission);
 
         WirePicker(panel, PickerAlbedo, painter ? painter.brushColor : Color.red,
-            c => { if (painter) painter.brushColor = new Color(c.r, c.g, c.b, painter.brushColor.a); });
+            c => { if (painter) painter.brushColor = c; });
 
         WireSlider(panel, SliderMetallic, painter ? painter.metallic : 0f, v => { if (painter) painter.metallic = v; });
         WireSlider(panel, SliderSmoothness, painter ? painter.smoothness : 0.5f, v => { if (painter) painter.smoothness = v; });
+        WireSlider(panel, SliderMetallicSmoothnessAlpha, painter ? painter.metallicSmoothnessOpacity : 1f, v => { if (painter) painter.metallicSmoothnessOpacity = v; });
         WireSlider(panel, SliderOcclusion, painter ? painter.occlusion : 0.5f, v => { if (painter) painter.occlusion = v; });
+        WireSlider(panel, SliderOcclusionAlpha, painter ? painter.occlusionOpacity : 1f, v => { if (painter) painter.occlusionOpacity = v; });
 
         WirePicker(panel, PickerEmission, painter ? painter.emissionColor : Color.black,
-            c => { if (painter) painter.emissionColor = new Color(c.r, c.g, c.b, painter.emissionColor.a); });
+            c => { if (painter) painter.emissionColor = c; });
         WireSlider(panel, SliderEmissionIntensity, painter ? painter.emissionIntensity : 1f, v => { if (painter) painter.emissionIntensity = v; });
 
         WireSlider(panel, SliderBrushSize, painter ? painter.brushSize : 16f, v => { if (painter) painter.brushSize = Mathf.RoundToInt(v); });
@@ -310,7 +314,7 @@ public class PaintHud : MonoBehaviour
     }
 
     // Cached gradient sprites for the colour picker.
-    private static Sprite _satSprite, _valSprite, _hueSprite;
+    private static Sprite _satSprite, _valSprite, _hueSprite, _alphaSprite;
 
     /// <summary>Horizontal white-out: opaque white (left) -> transparent (right) = saturation.</summary>
     public static Sprite SaturationSprite()
@@ -344,6 +348,23 @@ public class PaintHud : MonoBehaviour
         return _valSprite;
     }
 
+    /// <summary>Horizontal white transparency ramp: transparent (left) -> opaque (right) = alpha.
+    /// Tinted to the current colour by the picker so it reads as a colour-over-backdrop fade.</summary>
+    public static Sprite AlphaSprite()
+    {
+        if (_alphaSprite) return _alphaSprite;
+        const int n = 64;
+        var tex = NewGradientTex(n, 1);
+        for (int x = 0; x < n; x++)
+        {
+            byte a = (byte)Mathf.RoundToInt(255f * (x / (float)(n - 1)));
+            tex.SetPixel(x, 0, new Color32(255, 255, 255, a));
+        }
+        tex.Apply(false);
+        _alphaSprite = ToSprite(tex);
+        return _alphaSprite;
+    }
+
     /// <summary>Horizontal full-saturation hue ramp 0..1.</summary>
     public static Sprite HueSprite()
     {