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>

iso/overrides/packages.x86_64

@@ -19,10 +19,10 @@ snap-pac
 # boot splash
 plymouth
 
-# live installer host (cage). calamares is AUR -> rebuild into [tanin] first,
-# then uncomment (see BUILD.md). archinstall (TUI) is already in the releng base.
+# live installer: cage hosts calamares. calamares is AUR -> served from [tanin]
+# (rebuild via finish-tanin-repo.sh / the calamares one-liner in BUILD.md).
 cage
-#calamares
+calamares
 
 # target bootloader
 efibootmgr