fix: spline parse order (WotLK-first) fixes missing NPCs; bound WMO liquid loading

Spline auto-detection: try WotLK format before Classic to prevent false-positive
matches where durationMod float bytes resemble a valid Classic pointCount. This
caused the movement block to consume wrong byte count, corrupting the update mask
read (maskBlockCount=57/129/203 instead of ~5) and silently dropping NPC spawns.

Terrain latency: bound WMO liquid group loading to 4 groups per advanceFinalization
call. Large WMOs (e.g., Stormwind canals with 40+ liquid groups) previously loaded
all groups in one unbounded loop, blowing past the 8ms frame budget and causing
stalls up to 1300ms. Now yields back to processReadyTiles() after 4 groups so the
time budget check can break out.

include/rendering/terrain_manager.hpp

@@ -157,6 +157,7 @@ struct FinalizingTile {
     size_t wmoModelIndex    = 0;   // Next WMO model to upload
     size_t wmoInstanceIndex = 0;   // Next WMO placement to instantiate
     size_t wmoDoodadIndex   = 0;   // Next WMO doodad to upload
+    size_t wmoLiquidGroupIndex = 0; // Next liquid group within current WMO instance
 
     // Incremental terrain upload state (splits TERRAIN phase across frames)
     bool terrainPreloaded = false;  // True after preloaded textures uploaded