What changed

• Added fixture-backed API presence contracts:
  • core public API contract at tests/fixtures/conformance/api/public-api-v1.json
  • experimental preprocessor public API contract at tests/fixtures/preprocessor/public-api-v1.json
• Added focused tests that validate contract artifacts against Python exports:
  • tests/test_api_contract_fixture.py
  • tests/test_preprocessor_api_contract_fixture.py
• Updated preprocessor conformance fixture loading to ignore public-api-*.json files so behavioral fixture replay remains scoped to behavioral fixtures.
• Updated fixture docs to describe API presence contract intent and boundary from behavioral semantics.
• Bumped package version to 0.6.17 in pyproject.toml and uv.lock.

Why

• Existing cross-port fixtures enforced behavior but did not explicitly enforce required public API surface.
• A small artifact-based API contract enables ports (including TS) to consume required export/method presence directly while preserving existing behavioral conformance fixtures as the semantic source of truth.
• Version bump prepares this change set for release.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)