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.

src/game/game_handler.cpp

@@ -7329,6 +7329,17 @@ void GameHandler::sendMovement(Opcode opcode) {
         case Opcode::MSG_MOVE_HEARTBEAT:
             // No flag changes — just sends current position
             break;
+        case Opcode::MSG_MOVE_START_ASCEND:
+            movementInfo.flags |= static_cast<uint32_t>(MovementFlags::ASCENDING);
+            break;
+        case Opcode::MSG_MOVE_STOP_ASCEND:
+            // Clears ascending (and descending) — one stop opcode for both directions
+            movementInfo.flags &= ~static_cast<uint32_t>(MovementFlags::ASCENDING);
+            break;
+        case Opcode::MSG_MOVE_START_DESCEND:
+            // Descending: no separate flag; clear ASCENDING so they don't conflict
+            movementInfo.flags &= ~static_cast<uint32_t>(MovementFlags::ASCENDING);
+            break;
         default:
             break;
     }