Add deterministic route contract verification script

[Disaster-Terminator]

Motivation

• Provide a fast, deterministic local checker to detect route_id / target_model drift and misconfigured eval expectations before public readiness.
• Ensure route configuration references (fallbacks, hard_rules, sample expectations) are validated without any external embedding or LiteLLM calls.

Description

• Add scripts/verify_route_contract.py, a standalone verifier that defaults to config/routes.yaml and config/eval_cases.yaml and returns a non-zero exit on failure.
• The verifier checks that the route_model/entry_model is not used as a route_id, fallback_route_id/default_route exists, every hard_rules route_id exists, every route resolves to a non-empty target_model, and no resolved target_model equals the entry model, and that eval expect values reference route_id (not target_model).
• The script prints readable error lines for each problem and succeeds silently with a short success message when no issues are found.
• Add tests in tests/test_verify_route_contract.py including a passing config fixture and a failing fixture that exercise drift and missing-reference errors.

Testing

• Ran the full test suite with uv run python -m pytest -q and all tests passed: 152 passed, 1 warning (4.11s).
• Ran the verifier with uv run python scripts/verify_route_contract.py against repository configs and it succeeded with a pass message.
• Ran the evaluator with uv run python scripts/eval_routes.py --mock-embeddings and observed all sample eval cases pass (10/10).

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Codex Task

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