- Move run_preflight from build_container() into install_script() after header_info
- Shows: Header → preflight checks → 2s pause → clear (via next header_info call)
- On failure: show summary + exit cleanly (no ERR trap)
- Change all return 1 → return 0 in preflight functions (prevents ERR trap under set -Ee)
- Remove PREFLIGHT_DONE guard from build_container (no longer needed)