Add comprehensive documentation explaining return value semantics for malformed URIs. Changes: - Clarify that empty string indicates "unknown/unsupported collection" - Document that callers should validate return value before using for DB queries - Add examples of expected collection names (e.g., "social.coves.feed.comment") - Explain format expectations (at://did/collection/rkey) This addresses PR review feedback about input validation documentation. Empty string is the correct sentinel value indicating unparseable/invalid URIs, and callers must handle this appropriately. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>