Fixes#2028. A download that died mid-stream kept its partial file but started over from zero on retry. Downloads now resume with curl -C - when the partial belongs to the same URL (a .url sidecar records it; an untracked file or a different URL starts from zero, so a stale partial can't be spliced in); a server without byte ranges (curl exit 33) or HTTP 416 drops the partial and restarts. Covered by tests against a local HTTP server with and without Range support.
Co-authored-by: a-rookie-of-C-language <[email protected]>
