test(bench): add parity gate status tracker for #959

[DecisionNerd]

Summary

Adds an explicit #959 parity gate status reporter without fabricating ladder evidence.

• parity_gate_status() tracks each epic(bench): standardize BenchExec and Divan measurement and retire custom benchmark harnesses #959 acceptance criterion (tiny parity, historical readability, ladder bundle, retirement).
• make -C benchmarks parity-gate prints JSON status; exits non-zero only if tiny shadow parity fails.
• Fixes compare_ladder_bundle to select legacy migration fixtures by rung scale.
• Tests document that retirement remains blocked on test(scale): complete the S18-S26 lifecycle ladder on OVHC-AGENCY #900.

Testing

cd benchmarks
make parity-gate
PYTHONPATH=harness uv run --locked python -m unittest tests.test_parity_gate tests.test_scale_parity

Issue

Relates to #959 — legacy retirement still blocked until #900 Fly ladder bundles are ingested.

  

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