Add PVS (Potentially Visible Set) culling system

[Copilot]

Complete PVS culling system for offline visibility pre-computation and runtime occlusion culling.

Core Components

• PVSTypes.h - Cell/object IDs, grid coordinates, config structs
• PVSBitSet.h - SIMD-accelerated bitset for visibility storage with ForEachSetBit() fast iteration
• PVSData.h - 3D grid management with pre-computed inverse cell sizes for O(1) lookups
• PVSCulling.h - Runtime query interface with frustum culling integration

SIMD Acceleration

• SIMDUtils.h - Cross-platform (SSE/NEON/scalar) vectorized operations
• Bitwise OR/AND, popcount, batch AABB intersection, batch distance calculation
• 2-4x speedup on bitset operations

Baking Pipeline

• PVSBaker.h - Offline visibility computation (distance-based, raycast, custom)
• PVSBakedData.h - Serialization format with versioning and compression stats
• PVSSampling.h - Sample generation (random, stratified, Halton, Fibonacci sphere)

Large World Support

• PVSStreamingManager.h - Sector-based streaming with LRU eviction
• Automatic load/unload based on viewer distance
• Cross-sector visibility queries

BVH (lib core)

• BVH.h - Template-based acceleration structure with SAH build
• Ray/AABB/point queries
• Ray-AABB intersection in Shapes.h

Usage

// Bake visibility offline
PVSBaker baker;
baker.AddObjects(sceneObjects);
PVSBakedData data;
baker.Bake(config, data);
data.Save(archive);

// Runtime query
PVSCulling culling;
culling.LoadFromBakedData(data);
culling.ForEachVisiblePrimitive(cameraPos, view, [](auto* p) {
    // render
});

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