Complete archiso/Calamares profile scaffold (iso/)

- Calamares modules: unpackfs (clone), partition/mount/fstab (btrfs subvols),
  bootloader (systemd-boot + quiet splash), packages (strip live), services
  (greetd), shellprocess (snapper, flathub, strip-live), branding (Fuji)
- bootstrap-profile.sh: releng base + linux-zen swap + overlay our deltas
- BUILD.md: build steps, decisions, and honest open items (needs build-test)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/overrides/airootfs/etc/calamares/modules/bootloader.conf

@@ -0,0 +1,9 @@
+---
+efiBootLoader: "systemd-boot"
+kernel:   "/vmlinuz-linux-zen"
+img:      "/initramfs-linux-zen.img"
+fallback: "/initramfs-linux-zen-fallback.img"
+timeout:  "3"
+# Plymouth splash + quiet boot on the installed system.
+kernelParams: [ "quiet", "splash", "rw" ]
+installEFIFallback: true