Redesign Dialogue Lab case and turn identity

[RomanOstr]

Summary

• move canonical Case IDs to one global Case-NNN sequence
• make Case ID definitive while allowing multiple cases per Facebook root
• deduplicate Turns by reply_comment_id or the immutable fallback tuple
• rename the readiness command to dialogue-lab check and align CLI, SQLite v1 migration, skills, docs, and canonical workflows

Validation

• 51 pytest cases
• Ruff
• mypy
• JSON, governance history, and diff checks