feat(deploy): USB-bootable batm3 test image (disk-image-batm3-usb)

Add a USB-bootable BATM3 disk-image target plus the batm3 hardware
changes that make a dd'd USB stick boot reliably on the Dell 9030 AIO.

flake.nix — new `disk-image-batm3-usb` target:
- Distinct partition labels (nixos-usb / ESP-USB) so stage-1 by-label
  resolution can't latch onto an internal SATA drive that already holds
  a generic nixos/ESP-labelled install. Post-build mlabel relabels the
  ESP FAT volume to ESP-USB (bootloader files untouched; UEFI still
  loads /EFI/BOOT/BOOTX64.EFI).
- /boot mounted nofail + short device-timeout: the firmware already
  loaded the bootloader before Linux; without nofail a slow/late ESP-USB
  enumeration drops to emergency mode with root locked — a dead end.
- NO growPartition/autoResize on the USB image: 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
  uninterruptible D-state and ESP-USB vanishes with the device, so /boot
  times out too). Persistent state is a few MB and the image already
  ships ~2GB free in root. The internal-SATA disk-image-batm3 keeps
  growPartition — a real AHCI SSD won't drop the bus.
- autoUpgrade off (test image, not a managed fleet member).

batm3.nix — USB-boot reliability:
- Add usb_storage to initrd.availableKernelModules so stage-1 binds the
  stick and /dev/disk/by-label/* appears.
- Blacklist uas + usbcore.autosuspend=-1: force the slower-but-reliable
  Bulk-Only Transport path and stop the boot medium being power-suspended
  mid-I/O — both were causing "device offline error" bus drops.

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

deploy/nixos/hardware/batm3.nix

@@ -17,8 +17,22 @@
       "ahci"
       "usbhid"
       "sd_mod"
+      # USB mass-storage: required to boot the dd'd image from a USB stick
+      # (stage-1 must bind the flash drive as a SCSI disk so
+      # /dev/disk/by-label/nixos appears). Harmless on the internal-SATA
+      # install, where ahci+sd_mod already cover the root device.
+      #
+      # NOTE: deliberately NO "uas" here. Many USB sticks/bridges advertise
+      # UAS but drop off the bus ("device offline error, dev sdb") under the
+      # sustained write load of first-boot growPartition/journal/swapfile.
+      # Blacklisting uas below forces the slower-but-reliable usb-storage
+      # (Bulk-Only Transport) path. SATA/eMMC installs don't use uas anyway.
+      "usb_storage"
     ];
 
+    # Keep the USB flash drive off the flaky UAS driver (see note above).
+    blacklistedKernelModules = [ "uas" ];
+
     kernelModules = [
       "kvm-intel"
       "usbtouchscreen"
@@ -27,6 +41,9 @@
     kernelParams = [
       "quiet"
       "splash"
+      # Disable USB autosuspend so the boot medium (and kiosk peripherals)
+      # aren't power-suspended mid-I/O — another cause of "device offline".
+      "usbcore.autosuspend=-1"
     ];
   };