Fix main-thread hang from terrain finalization; two-pass M2 rendering; tile streaming improvements

Hang/GPU device lost fix:
- M2_INSTANCES and WMO_INSTANCES finalization phases now create instances
  incrementally (32 per step / 4 per step) instead of all at once, eliminating
  the >1s main-thread stalls that caused GPU fence timeouts and device loss

M2 two-pass transparent rendering:
- Opaque/alpha-test batches render in pass 1, transparent/additive in pass 2
  (back-to-front sorted) to fix wing transparency showing terrain instead of
  trees — adds hasTransparentBatches flag to skip models with no transparency

Tile streaming improvements:
- Sort new load queue entries nearest-first so critical tiles load before
  distant ones during fast taxi flight
- Increase taxi load radius 6→8 tiles, unload 9→12 for better coverage

Water refraction gated on FSR:
- Disable water refraction when FSR is not active (bugged without upscaling)
- Auto-disable refraction if FSR is turned off while refraction was on

include/rendering/m2_renderer.hpp

@@ -122,6 +122,7 @@ struct M2ModelGPU {
     bool isKoboldFlame = false;     // Model name matches kobold+(candle/torch/mine) (precomputed)
     bool isLavaModel = false;       // Model name contains lava/molten/magma (UV scroll fallback)
     bool hasTextureAnimation = false; // True if any batch has UV animation
+    bool hasTransparentBatches = false; // True if any batch uses alpha-blend or additive (blendMode >= 2)
     uint8_t availableLODs = 0;  // Bitmask: bit N set if any batch has submeshLevel==N
 
     // Particle emitter data (kept from M2Model)

include/rendering/terrain_manager.hpp

@@ -152,9 +152,11 @@ struct FinalizingTile {
     FinalizationPhase phase = FinalizationPhase::TERRAIN;
 
     // Progress indices within current phase
-    size_t m2ModelIndex = 0;       // Next M2 model to upload
-    size_t wmoModelIndex = 0;      // Next WMO model to upload
-    size_t wmoDoodadIndex = 0;     // Next WMO doodad to upload
+    size_t m2ModelIndex     = 0;   // Next M2 model to upload
+    size_t m2InstanceIndex  = 0;   // Next M2 placement to instantiate
+    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
 
     // Incremental terrain upload state (splits TERRAIN phase across frames)
     bool terrainPreloaded = false;  // True after preloaded textures uploaded