Add batch quantile queries to T-Digest

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.

Start by reviewing the existing quantile APIs on TDigest and TDigestMut, then compare PR #261's cursor, API, validation, tests, benchmark, and changelog changes. Verify that sorted ranks share one centroid traversal while unsorted input preserves caller order, and run the focused tests and benchmark without carrying over the merge changes.

Written by the indexing model from the issue text.