docs(pfi-gx10): AC power-restore patch applied, pending a real power-cut test; keep the original UEFI bytes

After the 2026-09-24 NH3 outage pfi-gx10 stayed off. It has no BMC, and the
ASUS firmware defaults to staying off after AC loss. With Prime's approval,
bytes 3-4 of the UEFI Setup variable were changed from 0x1d,0x00 to
0x1c,0x01, a community recipe that matches factory power-on units. The
readback shows exactly those two bytes changed. The original 85-byte raw
variable (attr header included) is committed here and also kept on the box.

The README records that an OS shutdown is not a test of this setting. It
stays off by design, and two such "tests" were invalid. The only valid test
is pulling and restoring AC, which Prime will do on 2026-09-25. It also
records the revert procedure.

servers/pfi-gx10/README.md

@@ -54,6 +54,37 @@ trivially while the desktop is still running.
 The playbook will not stop GDM while someone holds a seat session. Override with
 `--var force_dm_stop=true`, or just let the rack-install reboot handle it.
 
+## Power-on after AC loss — patched, NOT YET VALIDATED (2026-09-24)
+
+There is **no BMC**, and the ASUS firmware ships with "Restore AC Power Loss" set
+to stay off, so the box did not come back after the 2026-09-24 NH3 power outage.
+Prime approved patching the setting from the OS, headless:
+
+- UEFI var `Setup-ec87d643-eba4-4bb5-a1e5-3f3e36b20da9`, 81 bytes, attrs `0x7`,
+  BIOS `GX10DGX.0105.2026.0505.1153`. Bytes 3–4 changed `0x1d,0x00 → 0x1c,0x01`,
+  written with `efivar -w`. The readback shows exactly those two bytes changed.
+- The recipe is community-sourced (`Fheodoor01/DGX-Spark---Power-on-boot-`,
+  derived by diffing a power-on unit against a stay-off unit; verified by its
+  author on GX10DGX.0103/.0104). **Byte 3's meaning is not known.** The resulting
+  pair matches factory power-on units.
+- **Original bytes are saved twice:** `/var/lib/efi-setup-backup/Setup-ec87d643-20260924-pre-poweron.bin`
+  on the box (raw, including the 4-byte attr header) and
+  `servers/pfi-gx10/efi-Setup-ec87d643-20260924-pre-poweron.bin` here (sha256
+  `6d6f6f77…`). To revert, write `data[4:]` of that file back with
+  `efivar -w -n ec87d643-eba4-4bb5-a1e5-3f3e36b20da9-Setup -A 7 -f <file-without-header>`.
+
+⚠ **A shutdown is NOT a test of this setting.** "Power On after AC loss" means the
+box starts when power RETURNS after a cut. An OS `poweroff` is supposed to stay
+off, by design. The community README claims the two are indistinguishable. They
+are not on this board, and trusting that claim cost two trips to the power button
+(infra-ops, 1421 and 1435 on 2026-09-24). **The only valid test is pulling and
+restoring AC power without touching the button.**
+
+**Status at end of 2026-09-24:** the box is OFF (the 1435 shutdown), with the patch
+in place. Prime will pull and restore AC on 2026-09-25. It comes up on its own →
+the patch works, so delete "NOT YET VALIDATED" above. It stays off → press the
+button, then revert with the backup and set it in the firmware menu via a PiKVM.
+
 ## Training — run 3c is staged and ready
 
 The ERP-seat SFT LoRA (run 3c) that died on ana-ml2 at step 24 of 604 to an