fix: world map exploration fallback when server mask is unavailable

When the server has not sent SMSG_INIT_WORLD_STATES or the mask is
empty, fall back to locally-accumulated explored zones tracked by
player position. The local set is cleared when a real server mask
arrives so it doesn't persist stale data.

include/rendering/world_map.hpp

@@ -117,6 +117,8 @@ private:
     std::vector<uint32_t> serverExplorationMask;
     bool hasServerExplorationMask = false;
     std::unordered_set<int> exploredZones;
+    // Locally accumulated exploration (used as fallback when server mask is unavailable)
+    std::unordered_set<int> locallyExploredZones_;
 };
 
 } // namespace rendering