Implement distance-based LOD system for M2 models

Added LOD (Level of Detail) selection based on camera distance:
- 0-40 units: LOD 0 (base detail)
- 40-80 units: LOD 1 (medium detail)
- 80-150 units: LOD 2 (medium-low detail)
- 150+ units: LOD 3 (lowest detail)

Matches WoW retail behavior. Each M2 skin file contains multiple LOD
versions (submeshLevel 0-3). System filters batches to only render
the appropriate LOD for each instance's distance, significantly
improving performance with many distant models.

src/rendering/m2_renderer.cpp

@@ -1737,6 +1737,18 @@ void M2Renderer::render(const Camera& camera, const glm::mat4& view, const glm::
         for (const auto& batch : model.batches) {
             if (batch.indexCount == 0) continue;
 
+            // LOD selection based on distance (WoW retail behavior)
+            // submeshLevel: 0=base detail, 1=LOD1, 2=LOD2, 3=LOD3
+            float dist = std::sqrt(entry.distSq);
+            uint16_t desiredLOD = 0;
+            if (dist > 150.0f) desiredLOD = 3;       // Far: LOD3 (lowest detail)
+            else if (dist > 80.0f) desiredLOD = 2;   // Medium-far: LOD2
+            else if (dist > 40.0f) desiredLOD = 1;   // Medium: LOD1
+            // else desiredLOD = 0 (close: base detail)
+
+            // Skip batches that don't match desired LOD level
+            if (batch.submeshLevel != desiredLOD) continue;
+
             // Additive/mod batches (glow halos, light effects): collect as glow sprites
             // instead of rendering the mesh geometry which appears as flat orange disks.
             if (batch.blendMode >= 3) {