Adds workshop_paths.cache_path(steam_id) returning
$LEFT4ME_ROOT/workshop_cache/{steam_id}.vpk with digit-only validation.
Adds overlay_creation.generate_overlay_path(id) and
create_overlay_directory(overlay) with exist_ok=False so a stray dir from a
prior failed delete surfaces loudly instead of shadowing fresh content.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
