BufferPool: use best-fit (>=) instead of exact-size matching

[shinaoka]

Summary

strided-opteinsum/src/expr.rs BufferPool uses HashMap<usize, Vec<Vec<T>>> keyed by exact element count:

pool.f64_pool.get_mut(&total).and_then(|v| v.pop())

A buffer of 10001 elements is not reused for a request of 10000 elements, even though it would work fine.

Proposal

Use BTreeMap instead of HashMap, and find the smallest buffer >= requested size:

pool.f64_pool.range_mut(total..).next()

This improves cache hit rate, especially for contraction trees where intermediate sizes vary slightly.

Risk

Low. A larger buffer is always safe to use; only the first total elements are accessed.