Implement postprocessing, MultiTableMechanism.__call__ and output materialization in the relational synthesizer:

[copybara-service]

Implement postprocessing, MultiTableMechanism.call and output materialization in the relational synthesizer:

1. Decompression (_decompress_synthetic_datasets):

   • Probabilistically reverses categorical domain compression by sampling original category preimages from composite <Other> bins via mbi.Dataset.decompress.

2. Native TabularCodec Decoding (_decode_synthetic_tables):

   • Converts discrete integer bucket tokens back into continuous numerical floats (uniform interval dequantization) and categorical strings via TabularCodec.decode().

3. Relational Key Assignment (_assign_relational_keys):

   • Generates synthetic integer surrogate primary keys (0..N-1) for parent tables.
   • Vectorizes child foreign key assignment by indexing parent primary keys via parent_mappings from Phase 2 copula unstacking, guaranteeing 100% referential integrity without hash joins.

4. End-to-End Orchestration (MultiTableMechanism.call):

   • Connectsdiscretization & initializers, cascading discrete relational hierarchy synthesis & copula matching, and postprocessing & key linkage into MultiDataGenerationResult.

5. Testing & Optimization:

   • Added unit tests for decompression, decoding, multi-tier PK/FK assignment, and empty child edge cases.
   • Added end-to-end integration test verifying calibrated MultiTableMechanism execution with differential privacy guarantees and referential integrity.
   • Add parameterized subtests covering all eager MultiTableConfig.__post_init__ validation failures.

6. Eager MultiTableConfig.post_init Validation:

• Make foreign_keys a required argument with no default (= () removed).
• Enforce naming constraints

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