modkit: make the pass-threshold policy a parameter; rescue the human blood project

modkit_filter_mode selects the auto 10th-percentile threshold (default) or a fixed
threshold (modkit_filter_threshold, 0.75) for extract, pileup and summary. The fixed
mode exists because the archived human blood runs were called at 0.75 by the earlier
main-line code, and rescued pileups are only comparable with them under the same
policy. human_blood now runs the mitochondrial rescue (rescue_mt, GRCh38 NUMT loci)
with the fixed threshold; outputs are documented under
docs/pipeline_notes.md#numt_rescue.

Co-Authored-By: Claude Fable 5.1 <[email protected]>