One CPython interpreter caps sustained segment throughput at ~160 MB/s;
spawned children each run the existing thread-pool path on a contiguous
chunk of the batch, reaching NIC line rate on fast CDNs. Opt-in via
--download-processes (default 1), engaged only for 24+ segment batches.
Sessions are rebuilt per child from a picklable spec; RnetSession falls
back to a single process when it has no impersonate preset.
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