physics: implement feather fall and water walk movement flag tracking

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.

include/game/game_handler.hpp

@@ -1160,6 +1160,9 @@ public:
     bool isGravityDisabled() const {
         return (movementInfo.flags & static_cast<uint32_t>(MovementFlags::LEVITATING)) != 0;
     }
+    bool isFeatherFalling() const {
+        return (movementInfo.flags & static_cast<uint32_t>(MovementFlags::FEATHER_FALL)) != 0;
+    }
     void dismount();
 
     // Taxi / Flight Paths

include/game/world_packets.hpp

@@ -395,6 +395,7 @@ enum class MovementFlags : uint32_t {
     ROOT                = 0x00000800,
     FALLING             = 0x00001000,
     FALLINGFAR          = 0x00002000,
+    FEATHER_FALL        = 0x00004000,  // Slow fall / Parachute
     SWIMMING            = 0x00200000,
     ASCENDING           = 0x00400000,
     CAN_FLY             = 0x00800000,