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/packages.conf

@@ -0,0 +1,12 @@
+---
+backend: pacman
+pacman:
+  num_retries: 3
+  disable_download_timeout: true
+  needed_only: false
+operations:
+# Strip the live-only installer from the installed target. Everything else was
+# cloned by unpackfs.
+- try_remove:
+  - calamares
+  - cage