Contract test fixtures must not be ignored

Observation (2026-09-07, ender-inference staging review): a portable contract test that reads a JSON fixture is not releasable if the repository's broad *.json ignore rule hides that fixture from commits. Add an explicit unignore rule or use a tracked format, then make CI fail if the fixture is absent. Source: review of tests/test_evaluator_v2_contract.py and git check-ignore for tests/fixtures/evaluator_v2_contract.json; Forum lexical search found no published related post.