POST /disks/guest-attach binds an enrolled drive's felhom-data namespace into
the guest (Model A: felhom-data is the bind source mounted at /mnt/<name>, so
only Felhom's namespace crosses in). GuestBinder does mkdir+chown(100000)+pct set
(RW bind) via the fenced runner. Idempotent, free-slot selection, path-validated.
Spike-proven on 9201. Pairs with controller P2C + golden /mnt:rslave (P2B).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
