[tisonkun]

Add ddsketch::DdSketch behind the ddsketch feature for mergeable numeric quantiles with relative value accuracy, useful for heavy-tailed latency distributions. The implementation follows the DDSketch paper, with Datadog's implementation and the C++ proposal as references.

Fixed logarithmic buckets use bounded dense stores for positive and negative values. The default collapses the smallest numerical values to preserve high-quantile accuracy; callers can select highest-value collapse instead. quantile_is_collapsed identifies estimates that have lost the configured accuracy. The API includes integer weighted updates, compatible merging, exact extrema, allocation reuse, and lower discrete quantiles without losing rank precision above 2^53.

Subnormal values share the zero bucket and have an absolute error bound. Serialization uses a versioned Rust-specific format and preserves collapse state; it does not claim Datadog or TCK wire compatibility. Documentation, a distributed latency example, behavior and malformed-image tests, and update/merge/query/serialization benchmarks are included.

Validation:

• cargo x check, cargo x test, and cargo x lint.
• DDSketch behavior and serialization tests on Rust 1.86.0.
• ddsketch_latency example.
• DDSketch Divan benchmarks with --sample-count 20 --min-time 0.05 --max-time 0.2.