Commit 16adc5c silently dropped two defensive guards from the
symlink-creation loop in WorkshopBuilder.build. Restore them:
- refuse to overwrite a non-symlink file that collides with a workshop
name (logs a message, skips creation)
- refuse to overwrite a foreign symlink (target outside the cache root)
Also: change `skipped` from list to set (O(1) membership test, no
duplicates possible), and add a brief comment above WorkshopMetadata
construction explaining which fields download_to_cache actually uses.
Two regression tests added to pin the guard behaviour.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
81c6863cca