Name the upstream cause of the Debian 13 BIOS hang: Debian bug #1111240

The known-gaps entry, decision log, sample comment and builder comment
blamed the host; the cause is trixie's GRUB config ("terminal_output
gfxterm serial" with a serial console and no video device), reported
and reproduced upstream and still open. UEFI remains the default for the
reason recorded; bookworm images are noted as unaffected.

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

docs/architecture.md

@@ -455,8 +455,12 @@ they read as choices, not accidents. Chronological by build step.
   state at all; answering that with delete-and-recreate is too drastic,
   so the next poll decides.
 - **KVM guests boot UEFI by default, secure boot off.** Found on the
-  first real host: Debian 13's generic image hangs after GRUB under
-  SeaBIOS regardless of CPU model or seed, and boots under OVMF. Every
+  first real host: Debian 13's generic image hangs in GRUB under SeaBIOS
+  on a headless VM (Debian bug #1111240: `terminal_output gfxterm
+  serial` with a serial console and no video device) regardless of CPU
+  model or seed, and boots under OVMF with the serial console working.
+  The alternatives were a virtio console (loses the kernel's serial
+  output) or a video device the VMs have no use for. Every
   current distro cloud image supports UEFI, so it is the default and
   `firmware: bios` is the per-image escape; libvirt's firmware
   autoselection (`<os firmware="efi">`) keeps the OVMF path out of the