feat(deploy): USB-bootable douro image (disk-image-douro-usb)

The douro cutover to bitspire is being done remotely with a USB stick
and the machine's internal drive is not NixOS, so the stick has to be
the system rather than an installer medium. Give douro the same
run-from-USB shape batm3 already has.

flake.nix
- Lift the batm3-usb module and image post-processing into shared
  `usbBootModule` / `mkUsbDiskImage` helpers (distinct nixos-usb/ESP-USB
  labels, nofail /boot, no growPartition, autoUpgrade off, ESP relabel).
  batm3-usb evaluates to the same fileSystems/upgrade config as before.
- Add `nixosConfigurations.douro-usb` and
  `packages.disk-image-douro-usb` on top of douro-installed.

douro.nix
- Blacklist uas and set usbcore.autosuspend=-1, the same bus-drop
  hardening batm3.nix carries, so a stick is a reliable boot medium on
  the Bay Trail box.

README
- Document the -usb outputs and the flash-with-Etcher, no-installer flow.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

deploy/nixos/hardware/douro.nix

@@ -20,12 +20,24 @@
     initrd.availableKernelModules = [
       "xhci_pci"
       "ahci"
+      # 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/* appears). Harmless on the internal install.
+      #
+      # NOTE: deliberately NO "uas" here. Many USB sticks/bridges advertise
+      # UAS but drop off the bus ("device offline error, dev sdb") under
+      # sustained write load. Blacklisting uas below forces the slower-but-
+      # reliable usb-storage (Bulk-Only Transport) path. SATA/mSATA installs
+      # don't use uas anyway. (Same hardening as batm3.nix.)
       "usb_storage"
       "sd_mod"
       "sdhci_pci"
       "i915"
     ];
 
+    # Keep the USB flash drive off the flaky UAS driver (see note above).
+    blacklistedKernelModules = [ "uas" ];
+
     kernelModules = [
       "kvm-intel"
       "i2c-dev"
@@ -37,6 +49,9 @@
       "vt.handoff=7" # Bay Trail: preserve BIOS display init
       "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"
     ];
   };