armbian-install: document session log for 2026-06-07 boot recovery attempts

docs/armbian-install.md

@@ -172,4 +172,35 @@ For the rootless-first Armbian setup (default user created on first boot), `~/.x
 - eMMC multi-boot setup: `docs/emmc-multiboot.md`
 - Initial bring-up: `docs/initial-bringup.md`
 - Storage architecture: `docs/storage-architecture.md`
-- OPi 3B bring-up reference: `references/orangepi-3b-bringup.md` (in cyberdeck skill)
+- OPi 3B bring-up reference: `references/orangepi-3b-bringup.md` (in cyberdeck skill)
+
+---
+
+## Session Log — 2026-06-07 Boot Recovery
+
+### Background
+Orange Pi OS Arch was booting from eMMC (3-OS multi-boot layout, PARTITION_CONFIG=0x78). An Armbian 26.2.0 minimal (Debian Trixie, kernel 6.18.30) SD card was inserted to test booting. The Armbian image produced a black screen with underscore cursor on HDMI and hung. Upon removing the SD card, the eMMC no longer booted — same black screen / cursor symptom.
+
+### What Happened
+The Armbian SD card's u-boot wrote bootloader data to two places:
+1. **SPI NOR flash** — overwrote the previously-erased SPI chip, changing the boot priority back to SPI-first
+2. **eMMC bootloader sectors** (0-32767) — overwrote the existing idbloader at sector 64 and u-boot.itb at sector 16384 with Armbian's versions
+
+### Recovery Attempts
+
+| Step | Result |
+|------|--------|
+| SPI erase via maskrom (`rkdeveloptool ef`) | Success (resolved by switching to a better USB-A-to-A cable) |
+| eMMC bootloader rewrite — wrote Orange Pi OS's `idbloader.img` to sector 64 and `u-boot.itb` to sector 16384 from within SD-booted OPi OS | RKNS magic verified at sector 64 |
+| PARTITION_CONFIG check | **0x78** — verified with `mmc extcsd read` via `mmc-utils` installed on OPi OS Arch. The register was not changed. |
+| Full boot cycle (no SD) | Still hangs: black screen + underscore cursor |
+
+### Diagnostic Signals
+- **Green LED:** flashes a quick heartbeat pattern before the hang — the kernel starts booting (extlinux.conf is found, the kernel loads and begins executing) but doesn't complete init
+- The underscore cursor is the kernel's framebuffer console — it initialized fbcon but never reached userspace
+
+### Still Pending (next session)
+1. **Check extlinux.conf on eMMC p1** — mount `/dev/mmcblk0p1` from the SD-booted system and verify the `LINUX`, `INITRD`, `FDT` paths exist and `root=PARTUUID=` matches p2's PARTUUID
+2. **Check rootfs health on p2** — mount `/dev/mmcblk0p2` and verify filesystem integrity (look for corruption from all the power cycles / failed boots)
+3. **Check kernel + initrd file existence on p1** — the files that extlinux.conf references might have been deleted or corrupted during the Armbian u-boot write
+4. **Try a kernel panic capture** — add `panic=10` to the APPEND line in extlinux.conf to see if the kernel panics on a missing rootfs before the display driver initializes