Add free-camera mode with player-equivalent collision

Introduce a free-fly camera as the next entry in the Tab mode cycle
(Player Control -> Paint -> Free Cam):

- FreeCamController: dedicated CinemachineCamera plus a CharacterController
  mirroring the player capsule (radius 0.1, height 1.3, slope 59, step 0.15)
  so the free camera collides with walls/ceiling/floor exactly like the
  player. WASD/Q-E (and Space/Ctrl) fly relative to view; mouse looks.
- FreeCamMode: plain-C# GameMode mirroring PaintMode, with an Exit() that
  restores camera priority and re-enables player control + camera input.
- GameModeManager: register freeCam in the Tab cycle (gated like player).
- Scene: FreeCamera object wired, Main Camera custom blends (0.4s) for the
  vcam priority hand-off.

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

Assets/Scripts/FreeCamController.cs.meta

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