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>

packaging/build-tanin-repo.sh

@@ -17,7 +17,7 @@ mkdir -p "$OUT"
 
 OWN_SIMPLE=(tanin-greet tanin-libadwaita tanin-setup tanin-eww tanin-desktop)
 OWN_BUILD=(taninux)
-AUR_REBUILD=(eww-git tiramisu-git waypaper)
+AUR_REBUILD=(eww-git tiramisu-git waypaper calamares)
 
 build() {
   local p="$1"