Fix HLL union estimate/serialization regression from lazy KxQ rebuild

[SavicStefan]

Since the lazy KxQ/curMin rebuild was introduced ("skip updating kxq in HLL merge"), Hll8Array::mergeHll only sets rebuild_kxq_curmin_ and defers the recompute. Several consumers read that deferred state without honoring the flag, so unioning HLL sketches with different lgConfigK could:

• discard accumulated data: isEmpty() reads curMin_/numAtCurMin_, which stay at empty-sketch defaults after a downsampling merge, so a populated gadget looks empty and the next union update overwrites it;
• corrupt the estimate: internalCouponUpdate does incremental KxQ/HIP updates against the stale base;
• serialize non-deterministically: copyAs() rebuilds via register replay (convertToHll8) while estimate/bounds rebuild via the direct sum, so equivalent merges could serialize to different bytes.

The effect was estimates collapsing to ~one input's cardinality (merge-order dependent) and merge-order-dependent serialization; 4.0.1 had neither.

Fix, keeping the lazy-merge optimization:

• isEmpty(): a pending rebuild means the array is non-empty.
• internalCouponUpdate(): rebuild before an incremental update reads KxQ, only when the coupon changes a register (duplicate coupons stay lazy).
• copyAs(): make every same-type result use the direct-sum rebuild so all paths agree, keeping serialization merge-order independent.

Adds a regression test covering estimate order-independence, is_empty() after a downsampling merge, scalar-after-merge accumulation, and merge-order-independent serialization.