iso: calamares from [tanin] (AUR rebuild); document the build step

- packages.x86_64: calamares enabled (served from [tanin])
- finish/build-tanin-repo.sh: add calamares to the AUR rebuild list
- BUILD.md + manifest: calamares rebuild step + status

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

distro/camel.toml

@@ -144,6 +144,11 @@ name   = "waypaper"
 reason = "wallpaper setter (niri autostart: waypaper --restore)"
 status = "in [tanin]"
 
+[[packages.rebuilt_aur]]
+name   = "calamares"
+reason = "GUI installer for the ISO — AUR-only, not in official repos"
+status = "TODO — heavy Qt build; rebuild into [tanin] (finish-tanin-repo.sh / BUILD.md one-liner)"
+
 [[packages.rebuilt_aur]]
 name   = "paru (or paru-bin)"
 reason = "AUR helper — wanted preinstalled on the ISO; must be in [tanin] first"