build(bench): pin GDC identities and evidence contracts

[DecisionNerd]

Summary

• Add shared GDC pinned-identity, acquisition, suite-declaration, and sanitized evidence schemas under benchmarks/.
• Implement graphforge_bench.gdc_contracts validation that rejects incomplete provenance, checksum mismatch, missing assets, reference mismatch, and identity drift.
• Declare independently selectable GDC suites (Graphalytics, SNB Interactive/BI, FinBench Transaction, SPB inventory-only) with fixture-only checksummed stand-ins—no bulk datasets.

Closes #960

Test plan

• PYTHONPATH=harness uv run --locked python -m unittest tests.test_gdc_contracts -v (10 passed)
• PYTHONPATH=harness uv run --locked python -m graphforge_bench.smoke
• PYTHONPATH=harness uv run --locked python -m unittest discover -s tests -p 'test_*.py' (186 passed)
• CI Gate green at exact head SHA

Made with Cursor

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