Reduce IndexOrDocValuesQuery DV penalty from 8x to 6x

[costin]

Revisit the threshold in IndexOrDocValuesQuery for picking points vs DV (especially now with DocVlauesSkipper).
With block-level skipping, DV is competitive with points for selective leads in conjunctions. Reducing to 6x lets the DV path be chosen in the crossover regime, yielding 1.78x throughput on selective-lead queries without regressing any other case.

/4 was also tested but caused a 43% regression at 20% lead + 10M docs due to BKD cost estimation noise at
the threshold boundary

Benchmark

AMD EPYC 7R32 (c5a.2xlarge), JDK 25, 2 forks, 5 iters × 3s.
Index: monotonic timestamps + 100-bucket keyword field, forceMerge(1).
Queries: TermQuery lead (FILTER) + LongField.newRangeQuery (FILTER).

method	lead	range	docs	/8 (ops/s)	/6 (ops/s)	ratio
crossover10	10%	80%	1M	8,634 ± 66	15,362 ± 51	1.78x
crossover10	10%	80%	10M	285 ± 10	290 ± 1	1.02x
crossover20	20%	80%	1M	6,948 ± 141	6,835 ± 72	0.98x
crossover20	20%	80%	10M	183 ± 2	186 ± 0.4	1.02x
crossover30	30%	80%	1M	5,764 ± 47	5,832 ± 37	1.01x
crossover30	30%	80%	10M	72 ± 1	72 ± 1	1.01x
dvFavorable	1%	80%	1M	39,487 ± 161	39,658 ± 371	1.00x
dvFavorable	1%	80%	10M	4,508 ± 17	4,501 ± 18	1.00x
pointsFavorable	50%	5%	1M	4,693 ± 31	4,746 ± 18	1.01x
pointsFavorable	50%	5%	10M	2,350 ± 16	2,390 ± 13	1.02x

The crossover10 case (10% lead + 80% range, 1M docs) shows 1.78x: the reduced
divisor switches from points to DV, and DV with skip blocks is faster for this
selective-lead, broad-range pattern. All other cases show no regression.

Fix #16425

[utafrali]

The core one-line threshold change is well-justified by the included JMH benchmark and the reasoning that DocValuesSkipper narrows the DV-vs-points gap since the original 8x heuristic was introduced. Approving with a few suggestions to tidy up stale exploratory comments and small polish items in the benchmark.

[github-actions]

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