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/unpackfs.conf

@@ -0,0 +1,7 @@
+---
+# Clone the live squashfs onto the target. Path matches install_dir=tanin,
+# arch=x86_64, airootfs_image_type=squashfs (see profiledef.sh).
+unpack:
+- source: "/run/archiso/bootmnt/tanin/x86_64/airootfs.sfs"
+  sourcefs: "squashfs"
+  destination: ""