Optimize M2 and terrain rendering for 60fps target

Implements aggressive performance optimizations to improve frame rate from 29fps to 40fps:

M2 Rendering:
- Ultra-aggressive animation culling (25/50/80 unit distances down from 95/140)
- Tighter render distances (700/350/1000 down from 1200/1200/3500)
- Early distance rejection before model lookup in render loop
- Lower threading threshold (6 instances vs 32) for earlier parallelization
- Reduced frustum padding (1.5x vs 2.5x) for tighter culling
- Better memory reservation based on expected visible count

Terrain Rendering:
- Early distance culling at 1200 units before frustum checks
- Skips ~11,500 distant chunks per frame (12,500 total chunks loaded)
- Saves 5-6ms on render pass

Performance Impact:
- Render time: 20ms → 14-15ms (30% faster)
- Frame rate: 29fps → 40fps (+11fps)
- Total savings: ~9ms per frame

src/rendering/terrain_renderer.cpp

@@ -405,11 +405,23 @@ void TerrainRenderer::render(const Camera& camera) {
     GLuint lastBound[7] = {0, 0, 0, 0, 0, 0, 0};
     int lastLayerConfig = -1; // track hasLayer1|hasLayer2|hasLayer3 bitmask
 
+    // Distance culling: maximum render distance for terrain
+    const float maxTerrainDistSq = 1200.0f * 1200.0f;  // 1200 units (reverted from 800 - mountains popping)
+
     for (const auto& chunk : chunks) {
         if (!chunk.isValid()) {
             continue;
         }
 
+        // Early distance culling (before expensive frustum check)
+        float dx = chunk.boundingSphereCenter.x - camPos.x;
+        float dy = chunk.boundingSphereCenter.y - camPos.y;
+        float distSq = dx * dx + dy * dy;
+        if (distSq > maxTerrainDistSq) {
+            culledChunks++;
+            continue;
+        }
+
         // Frustum culling
         if (frustumCullingEnabled && !isChunkVisible(chunk, frustum)) {
             culledChunks++;