feat(hll): support all output types in union serialization #318

Problem

PR #317 adds HllUnion::serialize(), which directly emits an HLL8 result. The union accepts HLL4, HLL6, and HLL8 inputs, but callers choosing HLL4 or HLL6 output still need union.to_sketch(hll_type).serialize(), allocating an intermediate sketch. Applications persisting or transferring results should be able to choose the register representation when serializing directly.

Suggested approach

• Define an output-type selection API consistent with HllUnion::to_sketch(HllType); document the output representation and effective precision.
• Share encoding logic with sketch serialization. Keep the HLL8 fast path and implement HLL4/HLL6 output without constructing a complete temporary sketch merely to serialize it.
• Account for the packing and auxiliary-entry work required by HLL4/HLL6; direct serialization need not eliminate all scratch storage.

Start with datasketches/src/hll/union.rs, array4.rs, array6.rs, and array8.rs. This is a follow-up to #317; its current no-argument method is the starting point for the API discussion.

Validation

Compare each output type against to_sketch(type).serialize() for empty, LIST, SET, and dense results, including downsampling, merged estimator state, and HLL4 auxiliary entries. Verify round trips and existing cross-language fixtures. Extend benchmarks/hll/serde.rs to measure time and allocations for all three output types.

Follow CONTRIBUTING.md for changelog entries, integration-test placement, serialization fixtures, and the required checks.