Define CR-123 evaluation handoff contract

[coreytshaffer]

Summary

• Add the CR-123 evaluation handoff contract defining the file-based boundary between TriageCore and the external evaluator suite.
• Pin required fixture/actual inputs, contract identifiers, deterministic future bundle path vocabulary, and TriageCore-side exit-code expectations.
• Add focused documentation tests and link the existing fixture, actual-outcome, bridge, backlog, and changelog docs.

Scope Boundary

Scoring remains external to TriageCore. This PR does not add evaluator execution, score interpretation, model/backend calls, routing or admission integration, ledger writes, bundle building, or result importing/display.

Validation

• python -m pytest -q tests/test_eval_handoff_contract.py tests/test_eval_fixture_cli.py tests/test_eval_fixture_validator.py tests/test_eval_outcome_contract.py -> 32 passed
• python -m pytest -q -> 935 passed, 2 skipped