refactor(deploy): expose batm3-usb as a named nixosConfiguration

Lift the USB-variant config (distinct fs labels, nofail /boot, no
growPartition, autoUpgrade off) out of the inline disk-image-batm3-usb
`let` into `nixosConfigurations.batm3-usb`, and build the disk-image from
that same config. Enables in-place app deploys to a running stick via
`nix copy` + `switch-to-configuration` (build the toplevel, copy the
closure, activate) — no reflash, preserving pairing + /var/lib state.

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

flake.nix

@@ -292,6 +292,37 @@
         # at ttyJ5, dispenser at ttyJ7 layout) — reuse the same hw module.
         sintra-installed = mkInstalledConfig "sintra" ./deploy/nixos/hardware/upboard.nix;
         batm3-installed = mkInstalledConfig "batm3" ./deploy/nixos/hardware/batm3.nix;
+
+        # USB-bootable variant of batm3-installed. This is the config the
+        # flashed USB stick actually runs — distinct fs labels so stage-1 can't
+        # latch the internal drive, nofail /boot, no growPartition, autoUpgrade
+        # off. Exposed as a named config (not just inline in the disk-image
+        # target) so its system closure can be built here and deployed in-place
+        # with `nix copy` + `switch-to-configuration` — updating the app on a
+        # running stick WITHOUT reflashing (preserves pairing + /var/lib state).
+        # disk-image-batm3-usb builds its filesystem image from this same config.
+        batm3-usb = self.nixosConfigurations.batm3-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";
+              # /boot must NOT be a hard boot dependency on the USB image. The
+              # firmware already loaded the bootloader before Linux; without
+              # nofail, a slow/late ESP-USB enumeration (BOT is slower than UAS)
+              # blows past systemd's 90s device-timeout into emergency mode with
+              # root locked — a dead end. nofail + short timeout lets the
+              # already-mounted root carry the boot; /boot mounts if/when it shows.
+              fileSystems."/boot".options = [ "nofail" "x-systemd.device-timeout=10s" ];
+              # NO growPartition/autoResize: sfdisk rewriting the partition table
+              # on first boot is the single most bus-stressing write, and flaky
+              # USB bridges drop off the bus mid-rewrite (sfdisk wedges in D-state
+              # and ESP-USB vanishes with the device). Persistent state is a few
+              # MB and the image ships ~2GB free. The internal-SATA disk-image-
+              # batm3 keeps growPartition (a real AHCI SSD won't drop the bus).
+              system.autoUpgrade.enable = lib.mkForce false;
+            })
+          ];
+        };
       };
 
       # ── Standalone NixOS module ───────────────────────────────────
@@ -443,41 +474,12 @@
         # internal drive's ESP).
         disk-image-batm3-usb =
           let
-            cfg = self.nixosConfigurations.batm3-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";
-                  # /boot must NOT be a hard boot dependency on the USB test
-                  # image. The firmware already loaded the bootloader from the
-                  # ESP before Linux started; /boot is only remounted so the OS
-                  # can *update* the bootloader — which this image never does
-                  # (autoUpgrade off, no nixos-rebuild on the stick). Without
-                  # nofail, a slow/late ESP-USB enumeration (BOT is slower than
-                  # UAS) blows past systemd's 90s device-timeout and drops to
-                  # emergency mode — with root locked, an unrecoverable dead end.
-                  # nofail + a short timeout lets the (already-mounted) root carry
-                  # the boot to completion; /boot mounts if/when the ESP shows up.
-                  fileSystems."/boot".options = [ "nofail" "x-systemd.device-timeout=10s" ];
-                  # DELIBERATELY NO growPartition/autoResize on the USB image.
-                  # growPartition runs sfdisk to rewrite the stick's partition
-                  # table on first boot — the single most bus-stressing write of
-                  # the boot. Flaky USB bridges drop off the bus mid-rewrite
-                  # (sfdisk hangs forever as an uninterruptible D-state task) and,
-                  # worse, partition 1 (ESP-USB) vanishes with the device, so
-                  # /boot times out too. The kiosk's persistent state (state.db,
-                  # .env, wifi.conf, logs) is a few MB and the built image already
-                  # carries ~2GB free inside root — growing to fill the stick buys
-                  # nothing and costs reliability. The internal-SATA target
-                  # (disk-image-batm3) keeps growPartition: a real AHCI SSD won't
-                  # drop the bus and there filling the disk is worth it.
-                  system.autoUpgrade.enable = lib.mkForce false;
-                })
-              ];
-            };
+            # Filesystem image of the batm3-usb config (defined in
+            # nixosConfigurations). Same config that in-place deploys target, so
+            # a reflash and a `switch-to-configuration` converge on one system.
             baseImage = import (nixpkgs + "/nixos/lib/make-disk-image.nix") {
               inherit pkgs lib;
-              config = cfg.config;
+              config = self.nixosConfigurations.batm3-usb.config;
               format = "raw";
               partitionTableType = "efi";
               diskSize = "auto";