Replace the old skip-uncached-with-warning logic in WorkshopBuilder.build
with an inline download phase that calls _download_with_retry for each item
whose cache file is absent or stale (mtime/size mismatch). Stamps
last_downloaded_at / last_error after each download, and skips items with
no file_url. Update test fixture to utime cache files so mtime matches
time_updated, delete the now-superseded skip-warning test, and add six
new builder-level behavior tests covering the new download path.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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