Before invoking l4d2ctl initialize, run each blueprint overlay's builder
synchronously and then verify that every workshop item attached to the
blueprint has a cache file on disk. If any are missing, raise a clear
error naming the overlay and the missing steam_ids — server start can't
silently mount a partial overlay where some maps are mysteriously absent
in-game.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>