nx_pardl.nx

nx_pardl.nx -- PARALLEL fragment download (the performance capability yt-dlp/aria2 have via -N / --max-connection-per-server). Fork-pool: fetch `concurrency` fragments AT ONCE per batch (fork-per-fragment = the proven X-DLP idiom, and the per-fetch TLS mmap leak dies with each child), each child writes its fragment into a SHARED slot; the parent waits the batch, INTEGRITY-verifies each (nx_robust_dl), and marks the resume-state -- so parallelism composes with retry/integrity/resume, not instead of them. Measured, not checkboxed: pdl_download is timed serial-vs-parallel in the gate to REPORT the speedup axis.