BC-250 console image: custom GNOME recipe with remote admin

Based on 62fixolab/Latest-Bazzite-AMD-BC-250-Patched-Images @ 347fd4d.

Adds on top of the fork:
- recipes/bc250-console-gnome.yml: governor + gnome-remote-desktop +
  openssh-server, sshd enabled, hhd.service masked, no signing module
  (local build + ISO path)
- files/console/usr/bin/bc250-remote-setup: one-time on-box SSH/RDP setup
- files/console/usr/lib/bootc/kargs.d/50-bc250-ttm.toml: ttm memory kargs
- BUILD-CONSOLE.md: build -> ISO -> validation instructions

files/console/ is separate from files/system/ so the 40-CU unlock tooling
stays out of this stable 24-CU image.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

files/console/usr/lib/bootc/kargs.d/50-bc250-ttm.toml

@@ -0,0 +1,4 @@
+# BC-250: allow the GPU to use more than 8 GB of the 16 GB shared GDDR6.
+# 3959290 pages * 4 KiB ≈ 15.1 GiB ceiling for TTM allocations.
+# Harmless if Bazzite's hardware setup also applies these on first boot.
+kargs = ["ttm.pages_limit=3959290", "ttm.page_pool_size=3959290"]