Add owned batch merging to T-Digest

Aggregation pipelines often own multiple partial `TDigestMut` states. Repeated borrowed `merge` calls recompress intermediate results, so consuming an iterator of digests should combine them with one compression pass while retaining the existing borrowed API.

PR #261 explored `FromIterator`. A first implementation collected sorted slices into a `BinaryHeap` and then collected all centroids again, which added allocations and obscured the data flow. The revised approach lazily k-way merges fully compressed sorted buffers directly into compression; inputs with unsorted update tails are materialized and stably sorted once because sorting every tail separately was slower.

A focused PR should cover empty and single inputs, mixed `k` values, stable tie ordering, compressed and uncompressed inputs, allocation-aware benchmarks, and the clone tradeoff between borrowed `merge` and consuming collection.

Start by tracing the existing borrowed merge API and the TDigestMut compression path, then read PR #261 for the prior FromIterator approach and its allocation tradeoffs. Validate empty and single inputs, mixed k values, stable ties, and compressed or uncompressed inputs; add allocation-aware benchmarks and compare consuming collection with borrowed merge and cloning.

Written by the indexing model from the issue text.