Fix #89: make THINK vs SAY divergence observable

[DeveshParagiri]

Summary

• Add public_position to ReasoningResponse so high-fidelity reasoning can carry a separate public stance
• Classify private position from private_thought (fallback to reasoning), and classify public position from public_statement at high fidelity
• Apply response.public_position in engine state updates (fallback to response.position) so divergence persists to DB-visible state
• Include fidelity in meta.json for run auditability
• Add tests for async two-pass divergence classification, engine public-position override behavior, and fidelity metadata export

Testing

pytest -q tests/test_reasoning_execution.py tests/test_engine.py::TestFlipResistance::test_public_position_prefers_explicit_public_field tests/test_engine.py::TestTokenAccumulation::test_cost_in_meta_json

Closes #89

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Recreated from closed PR #96 (base branch was deleted)