feat(t400): add coreboot GRUB/UEFI boot variants and discrete ATI GPU

Split the T400 bootloader into self-contained, importable modules so the host
can match whatever firmware is flashed (switch by changing one import):
- boot-bios.nix       stock BIOS / coreboot+SeaBIOS -> GRUB on the MBR (default)
- boot-coreboot-grub.nix  coreboot GRUB payload -> config-only GRUB (device=nodev)
- boot-coreboot-uefi.nix  coreboot Tianocore/UEFI payload -> systemd-boot; carries
                          its own ESP (/boot vfat) so it travels with the mode

Cover the optional discrete ATI Mobility Radeon HD 3470 (RV620): load the open
`radeon` KMS driver in the initrd for early modesetting (firmware via
enableRedistributableFirmware), with a note on the T400's switchable graphics.

All three boot variants evaluate; nixfmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/T400/configuration.nix

@@ -5,15 +5,14 @@
 {
   imports = [
     ./hardware-configuration.nix
+    # Boot: import exactly ONE, matching the firmware currently flashed.
+    # Stock Lenovo BIOS and coreboot+SeaBIOS both use boot-bios.nix.
+    ./boot-bios.nix
+    # ./boot-coreboot-grub.nix # coreboot with the GRUB payload (config-only GRUB)
+    # ./boot-coreboot-uefi.nix # coreboot with the Tianocore/edk2 UEFI payload
+    #                          #   (systemd-boot; carries its own ESP mount)
   ];
 
-  # The T400 is legacy BIOS only (no UEFI), so boot via GRUB in BIOS mode.
-  # `device` must point at the actual install disk -- adjust if it is not /dev/sda.
-  boot.loader.grub = {
-    enable = true;
-    device = "/dev/sda";
-  };
-
   networking.hostName = "T400-NixOS";
 
   console.font = "Lat2-Terminus16";
@@ -32,12 +31,23 @@
   # lets swaylock authenticate via password.
   security.pam.services.swaylock = { };
 
-  # Intel Core 2 (Penryn) microcode + redistributable firmware for the Intel
-  # WiFi Link 5100/5300 iwlwifi blobs. The GMA 4500MHD works out of the box via
-  # i915/KMS, so no extra graphics config is needed.
+  # Intel Core 2 (Penryn) microcode + redistributable firmware. The latter also
+  # supplies the iwlwifi blobs (Intel WiFi Link 5100/5300) and the radeon
+  # firmware needed by the discrete GPU below.
   hardware.cpu.intel.updateMicrocode = true;
   hardware.enableRedistributableFirmware = true;
 
+  # This T400 has the optional discrete GPU fitted: an ATI Mobility Radeon HD
+  # 3470 (RV620), driven by the open `radeon` KMS driver. Load it in the initrd
+  # for early modesetting (clean Sway/Wayland start); firmware comes from
+  # enableRedistributableFirmware above.
+  #
+  # The T400 has switchable graphics (this discrete GPU + the Intel GMA
+  # 4500MHD). Select "Discrete" in the firmware's graphics setting so only the
+  # ATI is live; if you instead run "Integrated", the Intel i915 driver takes
+  # over with no extra config and `radeon` simply stays idle.
+  boot.initrd.kernelModules = [ "radeon" ];
+
   # See `man configuration.nix` / the stateVersion docs before changing.
   system.stateVersion = "26.05";
 }