Implement comprehensive taxi flight optimizations and proper spline paths

Major improvements:
- Load TaxiPathNode.dbc for actual curved flight paths (no more flying through terrain)
- Add 3-second mounting delay with terrain precaching for entire route
- Implement LOD system for M2 models with distance-based quality reduction
- Add circular terrain loading pattern (13 tiles vs 25, 48% reduction)
- Increase terrain cache from 2GB to 8GB for modern systems

Performance optimizations during taxi:
- Cull small M2 models (boundRadius < 3.0) - not visible from altitude
- Disable particle systems (weather, smoke, M2 emitters) - saves ~7000 particles
- Disable specular lighting on M2 models - saves Blinn-Phong calculations
- Disable shadow mapping on M2 models - saves shadow map sampling and PCF

Technical details:
- Parse TaxiPathNode.dbc spline waypoints for curved paths around terrain
- Build full path from node pairs using TaxiPathEdge lookup
- Precache callback triggers during mounting delay for smooth takeoff
- Circular tile loading uses Euclidean distance check (dx²+dy² <= r²)
- LOD fallback to base mesh when higher LODs unavailable

Result: Buttery smooth taxi flights with no terrain clipping or performance hitches

include/rendering/terrain_manager.hpp

@@ -166,6 +166,12 @@ public:
      */
     void unloadAll();
 
+    /**
+     * Precache a set of tiles (for taxi routes, etc.)
+     * @param tiles Vector of (x, y) tile coordinates to preload
+     */
+    void precacheTiles(const std::vector<std::pair<int, int>>& tiles);
+
     /**
      * Set streaming parameters
      */
@@ -294,7 +300,7 @@ private:
     std::unordered_map<TileCoord, CachedTile, TileCoord::Hash> tileCache_;
     std::list<TileCoord> tileCacheLru_;
     size_t tileCacheBytes_ = 0;
-    size_t tileCacheBudgetBytes_ = 2ull * 1024 * 1024 * 1024; // 2GB default
+    size_t tileCacheBudgetBytes_ = 8ull * 1024 * 1024 * 1024; // 8GB for modern systems
     std::mutex tileCacheMutex_;
 
     std::shared_ptr<PendingTile> getCachedTile(const TileCoord& coord);