Add semantic matching for joins specification

This spec defines the semantic matching behavior for DataJoint 2.0 joins:

- Introduces lineage tracking for attributes (origin table/column)
- Defines homologous vs non-homologous namesakes terminology
- Specifies matching rules: only homologous namesakes used for joins
- Documents ~lineage table architecture for lineage storage
- Includes dependency graph fallback for non-DJ schemas
- Covers all affected operations: *, &, -, aggr
- Removes @ operator in favor of .join(semantic_check=False)
- Updates dj.U behavior (& replaces * for table extension)
- Includes migration guide and testing strategy

Author: claude