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/InsectController.cs.meta

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