test: add checkpoint json conformance fixtures

[rlippmann]

What changed

• Added shared conformance checkpoint fixtures for checkpoint JSON wrapper behavior under tests/fixtures/conformance/checkpoint/.
• Added fixture coverage for:
  • export_checkpoint_json wrapper shape/parseability
  • import_checkpoint_json restore behavior
  • JSON round-trip parity across engines
  • pending-clarification restore and deterministic yes resolution
  • malformed/invalid checkpoint JSON rejection
  • all-or-nothing failure behavior for invalid JSON checkpoint imports
• Extended tests/test_fixtures.py checkpoint conformance action handling to support:
  • export_checkpoint_json
  • import_checkpoint_json
  • checkpoint_json_round_trip

Why

• Python had JSON-wrapper coverage in unit tests, but shared portable conformance fixtures for checkpoint JSON boundaries were missing.
• This hardens cross-language conformance so TypeScript 0.7 can target the shared checkpoint JSON contract directly.

Checklist

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