Many Jellyfin clients probe GET /emby/System/Info/Public during their
handshake. There was no /emby prefix anywhere in the tree, so that 404ed
and the client abandoned discovery and fell back to its manual "enter your
server URL" screen - the exact experience mDNS exists to remove.
Register installs every pattern twice through a mount helper, with the same
handler value both times, so the alias cannot drift from the root surface.
The JSON 404 fallbacks get /emby forms too, plus one subtree handler,
because static.go's isAPIPath knows only /healthz and /api/ and an unknown
/emby/... path would otherwise be served the SPA shell.
No new endpoint, no handler behaviour changed, and the bare root paths are
untouched: Jellyfin's own clients use them.
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