RELEASE_BASE_URL
Base URL for the stable and alpha firmware channels, one directory per version at the root: <version>/firmware-<target>-<version>.<ext>.
Artifacts are immutable once published — the version is part of every filename — so this host's long edge TTL is safe to cache against. Versions older than the published window are absent here, exactly as they are upstream; those fall back to the release zip named by the API's zip_url, which is a GitHub release asset and unaffected.