Note FydeOS image compatibility with install-flex-emmc.sh

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHROMEOS-FLEX.md

@@ -154,6 +154,49 @@ Two ways:
    With the USB card unplugged, the firmware falls through to `Internal
    eMMC` and boots Flex with no menu.
 
+## Other ChromiumOS-based images: FydeOS
+
+Checked 2026-09-27, by reading the image only (not installed):
+`FydeOS_for_PC_iris_v23.0-SP1-io.bin.zip` (board `amd64-fydeos_iris-io`,
+version `16700.56.23.51`), in `matt`'s `~/Downloads` (extracted copy in
+`~/fydeos-check`). **`install-flex-emmc.sh` should work with it
+unchanged.**
+
+- **Same installer image layout:** ROOT-A is p3 (2.7GB), ROOT-B is a
+  2MB placeholder, the ESP is p12. The same `chromeos-install` (Rust)
+  and `chromeos-install.sh`, `write_gpt.sh`, `cros_installer`,
+  `is_running_from_installer`, and coreutils `/bin/chroot` shim, so the
+  chroot wrapper applies.
+- `cros_installer` has the same `No recognized cros_XXX bios option`
+  check, so the fake `cros_efi` is still needed. The success line is the
+  same `Installation to '${DST}' complete.`
+- The target layout from `write_gpt.sh` is 2.66GB each for ROOT-A and
+  ROOT-B, with STATE filling the rest of the disk.
+- **FydeOS's `chromeos-postinst` adds steps:** it mounts the
+  destination's ESP (`${install_dev%[0-9]}12`) to sync a `cros_debug`
+  (dev-mode) flag in its `grub.cfg`, then runs
+  `/usr/share/dualboot/update_initrd.sh` if `rootdev` returns a loop
+  device, which it does in our chroot. That's harmless:
+  `update_initrd.sh` exits unless `fydeos_dualboot` is on
+  `/proc/cmdline`, which isn't in our faked cmdline, and other disks are
+  masked anyway.
+- The ESP uses GRUB (`/efi/boot/bootx64.efi` + `grub.cfg`, kernels in
+  `/syslinux/vmlinuz.{A,B}`) instead of Flex's `crdyboot`. The
+  `40_custom` chainload path still works. It's unsigned, which is fine
+  because Secure Boot is disabled on hp-chell (`mokutil --sb-state`).
+  The image's ESP UUID is `F35E-F13C`.
+
+Caveats before trying it:
+- It replaces Flex (same eMMC, full wipe).
+- The script rewrites the UUID in the `40_custom` entry titled
+  "ChromeOS Flex", so FydeOS would boot under that title. Rename the
+  entry, or make the script take its title from the image's
+  `lsb-release`. The script's messages and this doc also say "Flex"
+  throughout.
+- Hardware support is unverified: it's unclear whether the `iris` build
+  supports this 6th-gen Core m7-6Y75 / HD 515 (Skylake). Check FydeOS's
+  hardware compatibility list first.
+
 ## History
 
 - **2026-09-04**: first install by streaming the image with