Add in-game paint HUD bound to the mesh painter

Add a code-driven HUD that appears in paint mode and binds live to the
active MeshPainter: four channel toggles, HSV pickers for albedo and
emission, sliders for metallic/smoothness/occlusion, emission intensity
and brush size. The HUD self-assembles its own overlay canvas and event
system at runtime.

Add a ModeChanged event to GameModeManager so the HUD can follow the
active mode, and guard MeshPainter against painting through the UI.

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

Assets/Scripts/GameModeManager.cs

@@ -41,6 +41,10 @@ public class GameModeManager : MonoBehaviour
     public MeshPainter Painter => painter;
     public InsectController Insect => insect;
 
+    /// <summary>Raised after a mode switch finishes (the new mode's Enter has already run).
+    /// Argument = the now-active mode. Used by UI (e.g. the paint HUD) to follow the mode.</summary>
+    public event System.Action<GameMode> ModeChanged;
+
     private readonly List<GameMode> _modes = new List<GameMode>();
     private int _activeIndex = -1;
 
@@ -105,6 +109,7 @@ public class GameModeManager : MonoBehaviour
         ActiveMode.Enter();
 
         Debug.Log($"[GameModeManager] Mode -> {ActiveMode.DisplayName}", this);
+        ModeChanged?.Invoke(ActiveMode);
     }
 
     private bool WasSwitchPressed()