Fix WMO instance duplication causing 16x Stormwind rendering

Added deduplication for WMO instances based on uniqueId, matching the
existing M2 doodad deduplication logic. This prevents creating multiple
instances of the same WMO when it's referenced from multiple ADT tiles.

Before: STORMWIND.WMO (uniqueId=10047) was being rendered 16 times
        (one instance per ADT tile that references it)
After:  Only 1 instance is created and shared across all tiles

Changes:
- Added placedWmoIds set to TerrainManager (like placedDoodadIds)
- Check uniqueId before creating WMO instance
- Skip duplicate WMO placements across tile boundaries
- Log dedup statistics: 'X instances, Y dedup skipped'

This should fix the floating cathedral visual issue if it was caused by
rendering artifacts from 16x overdraw, and will massively improve
performance in Stormwind.

include/rendering/terrain_manager.hpp

@@ -323,6 +323,9 @@ private:
 
     // Dedup set for doodad placements across tile boundaries
     std::unordered_set<uint32_t> placedDoodadIds;
+
+    // Dedup set for WMO placements across tile boundaries (prevents rendering Stormwind 16x)
+    std::unordered_set<uint32_t> placedWmoIds;
 };
 
 } // namespace rendering