Upgrade the datasketches crate to 0.6.0 and drop the HLL decode workarounds #7395

What is the problem the feature request solves?

Comet pins the Rust datasketches crate at =0.3.0 with only the hll feature. That release has two HLL decode defects, and native/spark-expr/src/agg_funcs/hll_sketch.rs carries a workaround for each:

• Array4/Array6/Array8::deserialize skip the register block when the COMPACT flag is set, so a decoded sketch keeps its own HIP estimate but every union built from it is wrong. Comet clears the flag before decoding (normalize_compact_hll_array).
• An updatable HLL_4 sketch with auxiliary-map exceptions is read with the compact aux layout, so the exceptions are dropped and the estimate is silently wrong. Comet rejects that shape with an error (reject_undecodable_hll4), which means a sketch column written by DataSketches-Java with its default target type (HLL_4) can fail in hll_sketch_estimate, hll_union and hll_union_agg once it holds enough distinct values.

The crate's changelog lists fixes for both (compact HLL4 register restore in 0.5.0, the compact auxiliary-map flag in 0.4.0), plus HLL union estimate fixes that match Java and C++. 0.6.0 was released on 2026-10-29.

Describe the potential solution

Bump to =0.6.0 and delete both workarounds. Before deleting them, check the 0.6.0 decoder against DataSketches-Java 6.2.0 (the version Spark 4.1 ships) rather than trusting the changelog: serialize HLL_4, HLL_6 and HLL_8 sketches in compact and updatable form at several cardinalities, including ones with aux entries, and compare estimates and union estimates with Java's.

The API changes are small: hash_value moved to hash::value, and HllSketch::new / HllUnion::new now return Result.

Additional context

Follow-on work is tracked separately: an HLL audit against the new crate, and an epic for the sketch families the crate now provides (theta, KLL, frequent items, tuple).