feat(tdigest): Add owned batch merging

[ryux1]

Closes #269.

This is the focused owned-batch-merging half of #261, derived from @tisonkun's original implementation. The batch quantile API is intentionally excluded and tracked separately in #275.

This change:

• implements FromIterator with an owned k-way centroid merge
• preserves a single non-empty input without recompression and ignores empty inputs
• uses the smallest k across inputs and checks total-weight overflow
• materializes and stably sorts only inputs with uncompressed tails
• adds coverage for mixed k, empty/single inputs, compressed and uncompressed inputs, stable tie ordering, and overflow
• adds comparison benchmarks for repeated borrowed merging and consuming collection

Validation:

• cargo +nightly fmt --all --check
• cargo x check
• cargo x lint
• cargo x test
• cargo test --package tests-integration --test tdigest_test (26 passed)

Benchmark (100 samples, 64 partial digests):

• repeated merge median: 205.1 us, 53 allocations / 234.5 KB total
• FromIterator median: 58.23 us, 2 allocations / 131 KB total
• uncompressed FromIterator median: 156.3 us, 2 allocations / 131 KB total

Co-authored-by attribution to @tisonkun is preserved in the commit.