runtime: Support subdivision on managed threads.

[freezy]

• Replace per-subdivision Allocator.Temp arrays with reusable, allocator-owned buffers in NativeOctree<T> and NativeQuadtree<T>.
• Dispose the new buffers through both synchronous and job-dependent disposal paths.
• Add EditMode regression tests that force recursive subdivision from a plain managed thread.
• Enable package tests in the development project and document the allocator scope.

Motivation

VPE rebuilds its kinematic octree on a dedicated managed simulation thread. NativeTrees previously allocated subdivision scratch memory with Allocator.Temp, which Unity only permits on its main thread and job workers. Once a node exceeded objectsPerNode, subdivision threw an allocator exception.

The reusable buffer removes that thread restriction and avoids repeated allocations during recursive subdivision.