Download images from several mirrors at once with aria2c

Each *_IMAGE_URL may now list several mirrors of the same file. When
aria2c is installed, the file is split across all mirrors with a single
connection to each; otherwise curl or wget tries the mirrors in order,
skipping mirrors that fail or return an empty response. A partial
download is only resumed with the same downloader and mirror list.

Fedora now defaults to the TUNA, BFSU and NJU mirrors. The Aliyun
mirror is not used because it rejects aria2c.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YKxXLyiL7XJ3a9e8HmR2my

README.md

@@ -48,5 +48,13 @@ ISO (for example `Win11_25H2_English_x64_v2.iso` →
 existing `*Win*11*x64*.qcow2` in `base-qcow2/` is used instead.
 
 Download URLs can be overridden with `DEBIAN_IMAGE_URL`, `ALMA_IMAGE_URL`,
-`FEDORA_IMAGE_URL`, and `VIRTIO_WIN_ISO_URL`. The Fedora checksum can be
+`FEDORA_IMAGE_URL`, and `VIRTIO_WIN_ISO_URL`. Each may list several mirrors of
+the same file, separated by whitespace; the Fedora image defaults to the TUNA,
+BFSU and NJU mirrors.
+
+When [aria2](https://aria2.github.io/) is installed (`aria2c`), an image is
+downloaded from all of its mirrors at once, with a single connection to each
+mirror. Without it, `curl` (or `wget`) tries the mirrors in order. A partial
+download is only resumed with the same downloader and mirror list; otherwise
+it starts over. The Fedora checksum can be
 overridden with `FEDORA_IMAGE_SHA256` when using a custom image.