vdjtools 3.15.0

Added — one process pool, in one place

vdjtools.signature.cohort holds slices() and parallel_rows(): contiguous chunks, one task
per worker, spawn (polars cannot be combined with fork), and no serial fallback — a pool
that cannot start raises with the two fixes named. Three callers share it: vsig_cohort here,
rsig_cohort and signature_cohort in mirpy. Previously only mirpy had a pool and vdjtools had
none.

vsig_cohort gains n_jobs (default 1, 0 = every core this process may use) and accepts a
zero-argument callable per sample, which defers the read into the worker and keeps peak memory at
O(n_jobs) samples rather than the whole cohort.

Changed — the signature help no longer misdescribes mirpy

It said mir signature --preset classify ... "emits both halves as one vector". As of mirpy
3.18.0 it emits the geometry half only — one tool per half, joined on sample_id.