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/shellprocess-snapper.conf

@@ -0,0 +1,11 @@
+---
+dontChroot: false
+timeout: 120
+# Best-effort snapper setup on the Btrfs target. The @snapshots subvolume is
+# already mounted at /.snapshots (see mount.conf); snapper's own create-config
+# can conflict with a pre-existing /.snapshots, so this is tolerant.
+# snap-pac then auto-snapshots before/after each pacman transaction.
+script:
+- command: "snapper --no-dbus -c root create-config / || true"
+  timeout: 60
+- "snapper --no-dbus -c root set-config NUMBER_LIMIT=10 NUMBER_LIMIT_IMPORTANT=5 TIMELINE_CREATE=no || true"