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.

src/rendering/camera_controller.cpp

@@ -726,6 +726,9 @@ void CameraController::update(float deltaTime) {
                 else verticalVelocity *= std::max(0.0f, 1.0f - 3.0f * physicsDeltaTime);
             } else {
                 verticalVelocity += gravity * physicsDeltaTime;
+                // Feather Fall / Slow Fall: cap downward terminal velocity to ~2 m/s
+                if (featherFallActive_ && verticalVelocity < -2.0f)
+                    verticalVelocity = -2.0f;
             }
             targetPos.z += verticalVelocity * physicsDeltaTime;
             }