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/PaintMode.cs

@@ -8,8 +8,14 @@ public class PaintMode : GameMode
 
     public override void Enter()
     {
+        if (Manager.Insect)
+            Manager.Insect.enabled = false;
+
         if (Manager.Player)
+        {
+            Manager.Player.enabled = true;
             Manager.Player.ControlEnabled = false;
+        }
 
         if (Manager.CameraInput)
             Manager.CameraInput.LookMode = ThirdPersonOrbitInput.OrbitLookMode.HoldRightMouseButton;