gonic has arbitrary file write in createPlaylist: any authenticated user can write playlist M3U content to attacker-controlled path on the host

gonic is a music streaming server / free-software subsonic server API implementation. Prior to version 0.21.0, a logic error in ServeCreateOrUpdatePlaylist allows any authenticated Subsonic user (including non-admin) to write playlist M3U content to an attacker-controlled absolute filesystem path on the gonic host, and to create intermediate directories with 0o777 permissions. The bug is independent of CVE-2026-49338 and CVE-2026-49339. It is an unreachable guard clause combined with no path containment in Store.Write. Version 0.21.0 patches the issue.