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-grub.nix

@@ -0,0 +1,13 @@
+# Boot via coreboot's GRUB payload (e.g. libreboot default). The GRUB in the
+# flash chip reads the grub.cfg that NixOS generates on disk, so GRUB here is
+# config-only -- it is NOT installed to any disk MBR (`device = "nodev"`).
+#
+# Your coreboot grub.cfg must locate and load the on-disk config, e.g. search
+# for and `configfile` /boot/grub/grub.cfg (or chainload the disk's GRUB).
+{ ... }:
+{
+  boot.loader.grub = {
+    enable = true;
+    device = "nodev";
+  };
+}