fix: MSG_MOVE_START_DESCEND never set DESCENDING flag

Only ASCENDING was cleared — the DESCENDING flag was never toggled,
so outgoing movement packets during flight descent had incorrect flags.
Also clears DESCENDING on start-ascend and stop-ascend for symmetry.

Replaces static heartbeat log counter with member variable (was shared
across instances and not thread-safe) and demotes to LOG_DEBUG.

include/game/movement_handler.hpp

@@ -211,6 +211,7 @@ private:
     uint32_t fallStartMs_ = 0;
 
     // Heartbeat timing
+    int heartbeatLogCount_ = 0;  // periodic position audit counter
     float timeSinceLastMoveHeartbeat_ = 0.0f;
     float moveHeartbeatInterval_ = 0.5f;
     uint32_t lastHeartbeatSendTimeMs_ = 0;