physics: send MSG_MOVE_START/STOP_ASCEND and START_DESCEND during flight

When flyingActive_, detect Space/X key transitions and emit proper flight
vertical movement opcodes so the server (and other players) see the
correct ascending/descending animation state:

- MSG_MOVE_START_ASCEND  (Space pressed while flying)  → sets ASCENDING flag
- MSG_MOVE_STOP_ASCEND   (Space released while flying) → clears ASCENDING flag
- MSG_MOVE_START_DESCEND (X pressed while flying)      → clears ASCENDING flag
- MSG_MOVE_STOP_ASCEND   (X released while flying)     → clears vertical state

Track wasAscending_/wasDescending_ member state to detect transitions.
Also clear lingering vertical state when leaving flight mode.

include/rendering/camera_controller.hpp

@@ -256,6 +256,8 @@ private:
     bool wasTurningRight = false;
     bool wasJumping = false;
     bool wasFalling = false;
+    bool wasAscending_ = false;   // Space held while flyingActive_
+    bool wasDescending_ = false;  // X held while flyingActive_
     bool moveForwardActive = false;
     bool moveBackwardActive = false;
     bool strafeLeftActive = false;