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/rendering/camera_controller.hpp

@@ -97,6 +97,7 @@ public:
     void setMovementRooted(bool rooted) { movementRooted_ = rooted; }
     bool isMovementRooted() const { return movementRooted_; }
     void setGravityDisabled(bool disabled) { gravityDisabled_ = disabled; }
+    void setFeatherFallActive(bool active) { featherFallActive_ = active; }
     void setMounted(bool m) { mounted_ = m; }
     void setMountHeightOffset(float offset) { mountHeightOffset_ = offset; }
     void setExternalFollow(bool enabled) { externalFollow_ = enabled; }
@@ -279,6 +280,8 @@ private:
     bool movementRooted_ = false;
     // Server-driven gravity disable (levitate/hover): skip gravity accumulation.
     bool gravityDisabled_ = false;
+    // Server-driven feather fall: cap downward velocity to slow-fall terminal.
+    bool featherFallActive_ = false;
     bool mounted_ = false;
     float mountHeightOffset_ = 0.0f;
     bool externalMoving_ = false;