nas+nuc: cold boot verified; record what clearing the metadata errors took

Both hosts were powered off and brought back from a cold start, which is
the first real test of everything built on 2026-08-30.

nas: both pools imported from /etc/zfs/zpool.cache, all instances
autostarted, NFS exports republished, 0 failed units. nuc: /dev/dri
present with 0 i915 warnings (booted with the display unplugged), NFS
auto-remounted, VAAPI transcode at 9.5x realtime.

Records the ordering that actually cleared tank's inherited
<metadata>:<0x0> and <0x3d>: a scrub alone found 0 errors and repaired 0B
but left them, and a plain `zpool clear` afterwards did not drop them —
ZFS flushes the persistent error log on a scrub run *after* the clear.
That matters beyond tidiness, because while those entries stand
`zpool status -x` reports the pool unhealthy forever and zpool-health.sh
cannot signal anything new.

Two kiosk corrections, both from observed behaviour:

- the Pioneer DAC being switched off is the most likely cause of
  "video, no sound" — asound.conf pins the ALSA default to it by card
  name, so `default` fails to open outright and mpv falls back to null
  silently. Adds the one-line aplay check.
- hotplugging the display makes cage exit once and Restart=on-failure
  recovers it ~5s later. Do NOT restart it by hand; check
  ActiveEnterTimestamp against the hotplug time first.

Also flags that the OS mirror is still untested with a disk physically
unplugged — it is a guess until then.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

nas/nas-install.md

@@ -416,7 +416,30 @@ instances depend on that host surviving.
 
 Once `tank` has run a week on SATA with **zero** pool suspensions and no
 CRC errors, the `usb4t-dropouts` gate is cleared — record that in
-[ks2/plan.md](../ks2/plan.md).
+[ks2/plan.md](../ks2/plan.md). Day 1 was clean (2026-08-31).
+
+### Clearing the inherited `<metadata>` errors — order matters
+
+The pool imported carrying `<metadata>:<0x0>` and `<metadata>:<0x3d>` from
+the 2026-08-29 USB dropout. A scrub found **0 errors and repaired 0B**,
+yet the entries stayed, and a plain `zpool clear` afterwards did not drop
+them either. ZFS flushes its persistent error log on a scrub that runs
+**after** the clear — so the working order is:
+
+```sh
+zpool clear tank
+zpool scrub tank        # this is the run that flushes the log
+```
+
+Result 2026-08-31: `scrub repaired 0B in 02:34:24 with 0 errors`,
+`errors: No known data errors`, `all pools are healthy`. They were
+artefacts of interrupted writes, not corruption — matching the
+[2026-08-28 incident](../nuc/usb4t-dropouts.md).
+
+⚠️ This matters for monitoring, not just tidiness: while those entries
+stand, `zpool status -x` reports the pool unhealthy permanently, so
+`zpool-health.sh` sits in the alarm state and **cannot signal a new
+problem**. Clear them before trusting the watchdog.
 
 ## 6. Incus
 
@@ -804,11 +827,15 @@ trace can be recut against the meter's series afterwards.
 
 - [x] IPMI **left unplugged by decision** (2026-08-30) — no BMC on the
       LAN, therefore no out-of-band console either
-- [ ] `cat /proc/mdstat` — all three arrays `[UU]`; GRUB written to **both**
-      ESPs (`efibootmgr -v` shows two entries),
-      and boot tested with one disk unplugged
-- [ ] `zpool status` healthy for `incus` and `tank`; weekly scrubs
-      scheduled; both import cleanly after a reboot (`cachefile` set)
+- [x] `cat /proc/mdstat` — all three arrays `[UU]`; GRUB written to
+      **both** ESPs, both mounted, `debian` + `debian-b` boot entries
+      present (verified across a cold boot 2026-08-31)
+- [ ] boot still untested with **one disk physically unplugged** — the
+      mirror is a guess until that is done
+- [x] `zpool status` healthy for `incus` and `tank`; weekly scrubs
+      scheduled; **cold boot verified 2026-08-31** — both pools imported
+      from `/etc/zfs/zpool.cache`, all instances autostarted, NFS exports
+      republished, 0 failed units
 - [ ] **7 days with zero pool suspensions and zero CRC errors** — the
       gate that closes [usb4t-dropouts.md](../nuc/usb4t-dropouts.md)
 - [x] `smartd` monitoring all 3 disks; `zpool-health.sh -t` mail