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/boot-coreboot-uefi.nix

@@ -0,0 +1,17 @@
+# Boot via coreboot's Tianocore/edk2 (UEFI) payload. This turns the T400 into a
+# real UEFI machine, so use systemd-boot. Unlike Apple's firmware, coreboot's
+# UEFI honours EFI variable writes, so canTouchEfiVariables is on.
+#
+# Requires an EFI System Partition. It is declared here so it travels with this
+# boot mode; the generated hardware-configuration.nix should NOT also define
+# /boot. Label the ESP `ESP` at install, or replace with the generated UUID.
+{ ... }:
+{
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  fileSystems."/boot" = {
+    device = "/dev/disk/by-label/ESP";
+    fsType = "vfat";
+  };
+}