Close CR-124 eval handoff hygiene drift

[coreytshaffer]

Summary

• Fix write_actual_outcomes() so generator and other single-pass iterable inputs still write all actual outcome files after duplicate checks.
• Update stale eval fixture/taxonomy sequencing docs so they point from CR-123 toward bundle/manifest work instead of internal scoring.
• Add focused regression and drift coverage for the actual outcome writer and external-scoring boundary.

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, schema changes, bundle building, or result importing/display.

Validation

• python -m pytest -q tests/test_eval_outcome_contract.py tests/test_eval_handoff_contract.py tests/test_eval_fixture_cli.py tests/test_eval_fixture_validator.py -> 34 passed
• python -m pytest -q -> 937 passed, 2 skipped