Check Accept-Ranges before reaching for parallel download

aria2c only actually parallelizes when the server advertises Accept-Ranges: bytes; otherwise it silently falls back to a single stream. A quick curl -sI HEAD request reveals both Accept-Ranges and Content-Length in one round trip, so you can confirm parallelism is worth setting up before installing anything. Wrapping aria2c -x 16 -s 16 -k 1M -c as a script in ~/.local/bin makes it shell-agnostic across bash and zsh without editing either rc file, since the dir is already on PATH.