The AnimationPlayer correctly updates bone poses (proven in tests)
but the MeshInstance visual doesn't reflect the changes. Added
throttled _update_skin() that re-applies bone poses every ~33ms
to trigger NOTIFICATION_UPDATE_SKELETON on the Skeleton3D,
which causes the MeshInstance to update its vertex buffers.
Also set AnimationPlayer process_mode to ALWAYS to ensure it
processes regardless of parent node state.