docs: follow the storage move from nuc/USB to nas/SATA

backup-strategy (the entry point) still described nuc pulling the ks4
replicas: the leg, its WireGuard peer and the target pool now live on
nas with the 4 TB on direct SATA. Also: ks4 README flow chart redrawn
for the new topology, incus-copy leg 2 retargeted, usb4t-dropouts
marked RESOLVED (kept for the diagnosis method and the alerting gap),
and ks2/plan records that the interim push was deliberately not
re-enabled.

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

nuc/usb4t-dropouts.md

@@ -1,4 +1,13 @@
-# usb4t: USB dropouts suspend the pool (2026-08)
+# usb4t: USB dropouts suspend the pool (2026-08) — RESOLVED
+
+> **Outcome (2026-08-30): the disk moved off USB entirely.** It now
+> runs on **direct SATA** in the new host `nas`
+> ([nas/README.md](../nas/README.md)), as pool `tank`, and the ks4
+> pull leg plus its WireGuard tunnel moved with it
+> ([ks2/nas-seed.md](../ks2/nas-seed.md)). Everything below is the
+> investigation that led there — worth keeping for the diagnosis
+> method and for the alerting gap it exposed, which applies to any
+> host.
 
 Symptom seen first in the nightly backup log
 (`/var/log/incus-copy.log`, job in `/etc/cron.d/incus-copy`): every