test(hll): cover union rebuild regressions

[tisonkun]

Summary

• add regression coverage for the lazy estimator rebuild failure fixed by Fix HLL union estimate/serialization regression from lazy KxQ rebuild datasketches-cpp#512
• verify downsampling state, mixed-lg_k merge order, scalar updates after merge, and grouping-independent serialization
• exercise Hll4, Hll6, and Hll8 source representations

Testing

• cargo test --package datasketches --features hll --test hll_test
• cargo x check
• cargo x test
• cargo x lint

Upstream context: apache/datasketches-cpp#512