feat(deploy): add disk-image-sintra-usb with distinct partition labels

A USB-bootable Sintra image variant for booting on a machine whose eMMC
already holds a nixos/ESP-labelled install. make-disk-image hardcodes the
root/ESP labels (nixos/ESP); booting the standard image from USB next to
the eMMC races stage-1's by-label/nixos between the two roots and likely
mounts the eMMC. This variant labels root nixos-usb (via make-disk-image
-L) and relabels the ESP to ESP-USB in a post-step (mtools), with
fileSystems pointed at the new labels. Auto-upgrade is disabled — it's a
portable test / hand-off image, and that also removes scheduled bootloader
writes that could land on the eMMC's ESP.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.nix

@@ -326,6 +326,51 @@
           diskSize = "auto";
         };
 
+        # USB-bootable Sintra image with DISTINCT partition labels
+        # (nixos-usb / ESP-USB) so the stick can be booted on a Sintra whose
+        # eMMC already holds a nixos/ESP-labelled install without a by-label
+        # collision — stage-1 would otherwise race between the two roots and
+        # likely mount the eMMC. Auto-upgrade is disabled: this is a portable
+        # test / hand-off image, not a managed fleet member, and disabling it
+        # also removes the scheduled bootloader writes that could otherwise
+        # land on the eMMC's ESP.
+        disk-image-sintra-usb =
+          let
+            cfg = self.nixosConfigurations.sintra-installed.extendModules {
+              modules = [
+                ({ lib, ... }: {
+                  fileSystems."/".device = lib.mkForce "/dev/disk/by-label/nixos-usb";
+                  fileSystems."/boot".device = lib.mkForce "/dev/disk/by-label/ESP-USB";
+                  system.autoUpgrade.enable = lib.mkForce false;
+                })
+              ];
+            };
+            baseImage = import (nixpkgs + "/nixos/lib/make-disk-image.nix") {
+              inherit pkgs lib;
+              config = cfg.config;
+              format = "raw";
+              partitionTableType = "efi";
+              diskSize = "auto";
+              label = "nixos-usb"; # ext4 root label (make-disk-image -L)
+            };
+          in
+          pkgs.runCommand "nixos-disk-image-sintra-usb"
+            { nativeBuildInputs = [ pkgs.parted pkgs.mtools ]; }
+            ''
+              mkdir -p $out
+              cp --sparse=always ${baseImage}/nixos.img $out/nixos.img
+              chmod +w $out/nixos.img
+              # make-disk-image hardcodes the ESP FAT label to "ESP"; relabel the
+              # volume to ESP-USB so /boot (by-label/ESP-USB) doesn't collide with
+              # the eMMC's ESP. Volume label only — bootloader files are untouched,
+              # and UEFI loads /EFI/BOOT/BOOTX64.EFI regardless of the label.
+              espStart=$(parted -sm "$out/nixos.img" unit B print | awk -F: '$1==1 {gsub("B","",$2); print $2}')
+              echo "ESP partition starts at byte $espStart — relabelling to ESP-USB"
+              export MTOOLS_SKIP_CHECK=1
+              mlabel -i "$out/nixos.img@@$espStart" ::ESP-USB
+              printf 'verify ESP label: '; mlabel -i "$out/nixos.img@@$espStart" -s :: || true
+            '';
+
         # Backwards compat
         iso = self.nixosConfigurations.douro.config.system.build.isoImage;
       };