Add pipette (eyedropper) brush mode and wire it into the HUD

Pipette samples all painted channel values (albedo, metallic, smoothness,
occlusion, emission) at the clicked texel and adopts them into the brush
with every opacity forced to 1, instead of painting. The screen raycast is
extracted into a shared TryRaycastPaintMesh helper used by painting, preview
and the pipette. A pipette toggle is added to the HUD panel prefab and wired
in PaintHud.

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

Assets/Scripts/PaintHud.cs

@@ -48,6 +48,7 @@ public class PaintHud : MonoBehaviour
     private const string ToggleOcclusion = "Toggle_Occlusion";
     private const string ToggleEmission = "Toggle_Emission";
     private const string ToggleSpray = "Toggle_Spray";
+    private const string TogglePipette = "Toggle_Pipette";
     private const string PickerAlbedo = "Picker_Albedo";
     private const string PickerEmission = "Picker_Emission";
     private const string SliderMetallic = "Slider_Metallic";
@@ -169,6 +170,7 @@ public class PaintHud : MonoBehaviour
         WireToggle(panel, ToggleOcclusion, painter && painter.paintOcclusion, v => { if (painter) painter.paintOcclusion = v; });
         WireToggle(panel, ToggleEmission, painter && painter.paintEmission, v => { if (painter) painter.paintEmission = v; });
         WireToggle(panel, ToggleSpray, painter && painter.spray, v => { if (painter) painter.spray = v; });
+        WireToggle(panel, TogglePipette, painter && painter.pipette, v => { if (painter) painter.pipette = v; });
 
         // Dim a channel panel to 0.5 alpha while its toggle is off, full alpha while on.
         WirePanelFade(panel, PanelAlbedo, ToggleAlbedo);