v0.42.2 — fast cohort scans: shared construct cache, seeded target-BED depth

• perf(cohort): prepare_constructs builds the spliced-alignment cache with the same GTF, padding and shift settings as scan_sample, so scans hit the cache and never load the whole-genome index. The cache refreshes when the GTF changes (#133, #134).
• perf(depth): the sample median depth for --target-bed uses samtools bedcov -j on a seeded random subsample of target intervals (--target-bed-max-intervals, default 5000, 0 = all; --target-bed-seed, default 42), kept in BED order. This replaces the pure-Python base-level pass over every target.
• fix: --option=value overrides, an empty GTF, and depth-definition parity (quality_threshold=0).
• BIH benchmark (real exome BAM, 17 constructs, 8 threads): 4:28 per sample at 173 MB, versus more than 70 min (unfinished) on v0.42.1.