fix(query): value-weighted topk, HLL global rollup, range warm+archive stitch

[zzylol]

Three correctness fixes in the ASAP query backend (data_plane), each with a focused unit test. Full data_plane lib suite: 871 passed / 0 failed.

FIX 1 — CountSketch/CMS topk ranked by frequency (recall 0)

Root cause: the heavy-hitter heap (CountMinSketchWithHeapAccumulator) was built by +1-per-occurrence updates keyed by the raw item, so topk(k, sum by(label)(metric)) ranked groups by occurrence count, not summed value (eval-plan Fig 3c: "value-weighted topk needs a separate update path").
Change: count_min_sketch_with_heap_accumulator.rs — insert_value(group_label, value) adds the sample value (not +1) keyed by the group label (delegates to the library's value-weighted CountMinSketchWithHeap::update); topk_by_value(k) returns the top-k groups ranked by Σvalue.
Test: value_weighted_topk_has_full_recall_vs_count_topk — adversarial dataset where the busiest-by-count host (h_chatty, 100 tiny samples) is not the heaviest-by-value-sum host (h_heavy, few huge samples); asserts value-weighted top-k recall == 1.0 and excludes h_chatty. Plus insert_value_accumulates_summed_value_in_heap.

FIX 2 — HLL global rollup empty / per-series

Root cause: count(hll_metric) with no by (...) dispatched to the per-series Cardinality path, emitting one estimate per series (double-counting overlaps; never the single global number).
Change: sketch_reducer.rs::evaluate_cardinality_global merges per-series HLL registers (register-wise max) across all matched sids via the new delta_apply::cumulative_hll_state, then estimates once = distinct UNION cardinality. Wired in the instant execute path for CardinalityApprox + empty group_by + outer Count.
Test: execute_count_hll_global_merges_registers_across_series — two HLL series with overlapping + disjoint items; asserts the merged global estimate is within HLL error of the true union (1000) and well below the naive per-series sum (~1200).

FIX 3 — Range queries didn't hybrid-stitch warm+archive

Root cause: the instant path stitches warm+archive when warm coverage is narrower than the request, but execute_range_promql_modern returned warm-only, losing the prefix [start, cov_lo).
Change: wire the same coverage-aware stitch into the range path — when the reducer reports a coverage narrower than [start_ms, end_ms] AND an archive engine is configured, fetch the archive range answer and stitch via stitch_warm_and_archive (warm wins on overlap).
Test: range_stitches_archive_prefix_with_warm_suffix — warm (CountMin count_over_time) covers only the suffix; asserts the stitched matrix spans the full range (prefix from archive, suffix from warm, warm wins on overlap). Plus range_warm_only_when_no_archive_engine control.

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