Background AppImage launches executed directly from a FUSE mount owned by a
short-lived helper's AppImage runtime. On quit, the runtime unmounted the
squashfs while utility children (network service) were still mid-shutdown,
SIGBUSing them and triggering KDE Service Crash notifications on every video
close. Detach a POSIX-sh supervisor instead: mount via --appimage-mount, run
AppRun from the mount, and release the holder only once no process still
executes from the mount. Falls back to the old direct launch on any failure
or with SUBMINER_NO_APPIMAGE_MOUNT_KEEPALIVE=1.
