Add insect surface-walker controller and mode

Add InsectController (transform-driven SphereCast surface following, no
CharacterController/Rigidbody, no gravity or slope limit) and InsectMode as a
third game mode. Wire it into GameModeManager with conditional registration so
PlayerControl and Insect modes only join the Tab cycle when their controller is
present. PaintMode/PlayerControlMode toggle the insect controller accordingly.

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

Assets/Scripts/InsectMode.cs

@@ -0,0 +1,38 @@
+/// <summary>
+/// Insect / surface-walk mode: the player crawls along any surface (walls, ceilings,
+/// arbitrary slopes) via <see cref="InsectController"/>. The gravity-based
+/// <see cref="PlayerCharacterController"/> is disabled so the two never fight. The
+/// camera keeps its world-up orbit; painting is off.
+/// </summary>
+public class InsectMode : GameMode
+{
+    public override string DisplayName => "Insect";
+
+    public override void Enter()
+    {
+        if (Manager.Player)
+            Manager.Player.enabled = false; // stop the gravity controller (its Update/gravity)
+
+        if (Manager.Insect)
+        {
+            Manager.Insect.enabled = true;
+            Manager.Insect.ControlEnabled = true;
+            Manager.Insect.SnapToSurface();
+        }
+
+        if (Manager.CameraInput)
+            Manager.CameraInput.LookMode = ThirdPersonOrbitInput.OrbitLookMode.AlwaysActive;
+
+        if (Manager.Painter)
+            Manager.Painter.PaintingEnabled = false;
+    }
+
+    public override void Exit()
+    {
+        if (Manager.Insect)
+            Manager.Insect.enabled = false;
+
+        if (Manager.Player)
+            Manager.Player.enabled = true;
+    }
+}