Add eval fixture validation CLI (CR-122)

[coreytshaffer]

Scope

Adds a tc eval validate-fixtures --input <path> command that wraps the CR-121 pure JSONL fixture validator.

Included

• CLI success and fail-closed handling for eval fixture validation
• Line-aware diagnostic output for malformed or contract-invalid JSONL
• Missing-file/read-error handling without invoking eval execution
• Synthetic CLI coverage and fixture-contract documentation updates

Excluded

No tc eval scoring changes, model/backend calls, routing/admission integration, ledger writes, probe execution, or adversarial/tampering expansion.

Validation

• python -m pytest -q tests/test_eval_fixture_cli.py tests/test_eval_fixture_validator.py -> 15 passed
• python -m pytest -q tests/test_eval_fixture_cli.py tests/test_eval_fixture_validator.py tests/test_tc_cli.py tests/test_eval_review_cli.py -> 37 passed
• python -m pytest -q -> 931 passed, 2 skipped
• git diff --check clean
• git show --check HEAD clean