Introduces the dpsynth.relational submodule skeletons, BUILD rules, and interface contracts for differentially private multi-table relational synthesis.

[copybara-service]

Introduces the dpsynth.relational submodule skeletons, BUILD rules, and interface contracts for differentially private multi-table relational synthesis.

Key additions:

• Wrote third_party/py/dpsynth/relational/README.md with short announcement about active development and API.
• Created third_party/py/dpsynth/relational/BUILD with pytype_strict_library rules and test targets.
• Defined the ForeignKeyRelation dataclass in domain.py with group capacity bounds and cascading sensitivity documentation.
• Established strict function and method signatures for topological DAG sorting, schema parsing, pure relational transformations, and the MultiTableConfig / MultiTableMechanism abstractions matching the latest dpsynth architecture.
• Added initial unit test suites across domain_test.py, transformations_test.py, and synthesizer_test.py.
• Exported public symbols in third_party/py/dpsynth/relational/init.py.

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