nuc: document the usb4t dropout incident, recovery and alerting fix

Nightly replication had been failing since Aug 17 behind a misleading
incus error; root cause is a JMicron USB bridge dropping off the bus
(61 disconnects in 30 days) and suspending the pool — the disk itself
is SMART-clean. Records the recovery (clear/scrub), the damage, the
ordered fixes, and why nothing alerted: zed was running without an MTA
on the host.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nuc/README.md

@@ -10,6 +10,9 @@ Incus host on the LAN.
   `usb4t/media` → `/srv/media` (media library, shared into containers
   via `shift=true` disk devices; works because ZFS ≥ 2.2 supports
   idmapped mounts)
+- ⚠️ The USB enclosure drops off the bus periodically and suspends the
+  pool, which silently breaks the nightly replication — symptoms,
+  recovery and fixes: [usb4t-dropouts.md](usb4t-dropouts.md)
 - Backups: all local instances replicated to the USB pool
   (`/root/scripts/incus-copy.sh -p backup -s nucbackup`; replicas
   stopped, autostart off) — see