Add first-person camera mode with pose-aware eye offset and yaw facing

Introduce a FirstPersonController / FirstPersonMode pair (mirroring the
free-cam mode) wired into the GameModeManager Tab cycle. In first person the
mesh is hidden, the eye sits on CameraTarget plus a per-pose
firstPersonCameraTargetYOffset, and the character body yaw is driven to match
the camera via a new single-writer FaceYaw hook on PlayerCharacterController.

Also includes user content edits (poses, prefabs, scene) carried along.

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

Assets/Poses/Pose_Ball.asset

@@ -22,3 +22,4 @@ MonoBehaviour:
   controllerRadius: 0.15
   controllerHeight: 0.5
   cameraTargetYOffset: -0.5
+  firstPersonCameraTargetYOffset: -0.8

Assets/Poses/Pose_Sit.asset

@@ -22,3 +22,4 @@ MonoBehaviour:
   controllerRadius: 0.1
   controllerHeight: 0.85
   cameraTargetYOffset: -0.2
+  firstPersonCameraTargetYOffset: -0.2

Assets/Poses/Pose_Star.asset

@@ -22,3 +22,4 @@ MonoBehaviour:
   controllerRadius: 0.1
   controllerHeight: 0.3
   cameraTargetYOffset: -0.55
+  firstPersonCameraTargetYOffset: -1.18