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>

distro/MANIFEST.md

@@ -83,7 +83,7 @@ waypaper/cliphist) are exactly why `[tanin]` must carry rebuilt AUR packages.
    package time; `tanin-setup` seeds them on first run.
 3. **`[tanin]` repo** — `repo-add -s` to build+sign the DB. Host as a
    Gitea/Forgejo Arch registry **or** a static dir behind `taninux.kgva.ch`.
-4. **ISO** — archiso profile with `[tanin]` pre-configured; Calamares `packages`
+4. **ISO** — ✅ scaffolded in `iso/` (archiso releng-base + overrides). Calamares `packages`
    module installs `tanin-desktop`; greetd + tanin-greet enabled; session target wired.
 
 ### Session / dark-mode fix