Add signal store, DQ checks, and SQL lineage parser guides - #27

Summary

• Add docs/signal-store.md: setup, execution context, cascading deletes, DQ result storage, custom implementations
• Add docs/dq-checks.md: 5 built-in Polars checks, failure modes, check reports, writing custom checks
• Add docs/sql-lineage.md: source/sink extraction, CTE handling, dialect support, Airflow integration
• Add all three to mkdocs nav under Guides

Test plan

• mkdocs build --strict passes
• All code examples match actual API signatures
• No new dependencies