Fix out-of-bounds end index in SphereBVH::fromScene

[Copilot]

SphereBVH::fromScene passed entities.size() as the end argument to buildBVH, but entities lacking a MeshRendererComponent are skipped, so spheres can be smaller than entities. This causes buildBVH to operate past the end of the spheres vector.

Change

• src/physics/SphereBVH.cpp: Replace entities.size() with spheres.size() in the buildBVH call.

// Before
return SphereBVH(buildBVH(spheres, 0, entities.size()));

// After
return SphereBVH(buildBVH(spheres, 0, spheres.size()));

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