refactor: tighten KLL API and query performance

[tisonkun]

Summary

• rename KLL internals around their responsibilities and align visibility with the rest of the crate
• encode item ordering in Ord, support custom orderings through newtypes, and provide KllFloat for non-NaN floating-point values
• align KLL queries with REQ through shared SearchCriteria, explicit inclusive/exclusive selection, fallible validation, batch quantiles, and a reusable SortedView
• reject invalid merges before mutation and improve invariant diagnostics
• cache retained/capacity state, avoid unnecessary sorted views, merge sorted levels directly, and streamline compaction
• standardize truncated-input checks and expected/available byte diagnostics across sketch deserializers
• reorganize deterministic KLL coverage, enable Go compatibility fixtures, and add update, query, merge, and serialization benchmarks

Breaking changes

• import SearchCriteria from datasketches::common and select Inclusive or Exclusive explicitly
• custom KLL item types now implement Ord and KllValue; floating-point sketches use KllFloat<f32> or KllFloat<f64>

Validation

• cargo x check
• cargo x test
• cargo x lint
• cargo bench -p benchmarks --no-run