feat(conductor): add TPU production evidence contract

[edithatogo]

Summary

• add a machine-readable TPU production-scale evidence schema and validator
• add deterministic CPU reference and explicit TPU Colab/gcloud blocked packets
• document compile/transfer overhead, EVPI parity, CPU fallback, and the external runtime gate
• register Track 31 as the next executable Conductor track

Verification

• uv run pytest tests/test_tpu_speedup_evidence.py tests/test_conductor_followthrough_tracks.py tests/test_hpc_evidence_docs.py tests/test_registry_audit.py --no-cov (27 passed)
• uv run ruff check scripts/validate_tpu_speedup_evidence.py tests/test_tpu_speedup_evidence.py
• uv run ruff format scripts/validate_tpu_speedup_evidence.py tests/test_tpu_speedup_evidence.py --check
• Astro check/build passed locally

[gemini-code-assist]

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