Add batch quantile queries to T-Digest

[tisonkun]

Repeated quantile calls scan the same centroid sequence independently. T-Digest should expose batch queries on TDigest and TDigestMut so ranks in nondecreasing order share one traversal while arbitrary input order is preserved.

PR #261 explored this with a small cursor over the centroid sequence: sorted ranks use it directly, while unsorted ranks sort indices and scatter answers back into caller order. A focused PR should carry over the API, validation, tests, benchmark, and changelog entry without the merge changes from #261.

[ryux1]

I can take the focused batch-quantile half of #261. I’ll preserve attribution to the original implementation, isolate TDigestMut::quantiles / TDigest::quantiles, validation, order-preserving tests, the query benchmark, and a focused changelog entry, without carrying over the owned-merge changes.