Feather Fall (SMSG_MOVE_FEATHER_FALL / SMSG_MOVE_NORMAL_FALL):
- Add FEATHER_FALL = 0x00004000 to MovementFlags enum
- Fix handlers to set/clear the flag instead of passing flag=0
- Cap downward terminal velocity at -2.0 m/s in CameraController when
feather fall is active (Slow Fall, Parachute, etc.)
All three handlers now correctly propagate server movement state flags
that were previously acknowledged without updating any local state.