iso: cage needs a seat -> add+enable seatd (+ qt6-wayland for calamares)

VM test showed: boot menu (TANINUX branding ✓), root autologin ✓, motd ✓,
then cage failed: libseat 'Could not connect to /run/seatd.sock'. The getty
autologin has no logind seat, so run seatd in the live (disabled on the target,
where greetd uses logind). qt6-wayland so calamares (Qt) runs under cage.

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

iso/overrides/packages.x86_64

@@ -23,6 +23,11 @@ plymouth
 # (rebuild via finish-tanin-repo.sh / the calamares one-liner in BUILD.md).
 cage
 calamares
+# seatd: cage/wlroots needs a seat; the getty-autologin has no logind seat, so
+# run seatd (disabled on the target — greetd uses logind there).
+seatd
+# qt6-wayland: calamares (Qt) needs the wayland platform plugin to run under cage
+qt6-wayland
 
 # resolve provider prompts non-interactively (mkarchiso can't answer them):
 # audio -> pipewire's JACK (matches the pipewire stack); netfilter -> iptables(-nft)