feat(tdigest): Add batch quantile queries

[ryux1]

Summary

• add TDigestMut::quantiles and TDigest::quantiles
• reuse a forward-only centroid cursor for nondecreasing ranks
• preserve caller order for arbitrary rank input by sorting indices
• add validation, integration coverage, benchmarks, and changelog entries

This is the focused batch-query half requested after #261 was split. The implementation is derived from @tisonkun’s work in #261; the commit preserves that contribution with a Co-authored-by trailer. It intentionally excludes all owned batch-merge changes.

Closes #268.

Validation

• cargo +nightly fmt --all --check
• cargo x check
• cargo x prepare-testdata
• cargo x test
• cargo x lint
• cargo bench --package benchmarks --bench benchmarks -- tdigest::query::quantiles --sample-count 100

The local benchmark measured six sequential scalar queries at a 2.409 us median and the six-rank batch at a 297 ns median.