test: add portable serialization and checkpoint fixtures

[rlippmann]

What changed

• Added portable core-engine conformance fixture categories for serialization and checkpoint behaviors:
  • tests/fixtures/conformance/state-json/
  • tests/fixtures/conformance/checkpoint/
• Added deterministic fixture cases for:
  • state JSON export canonicalization and import validation boundaries
  • checkpoint import validation boundaries and pending-clear semantics
• Added two step fixtures for pending confirmation token normalization with punctuation:
  • 018_pending_affirmative_punctuation_token.json
  • 019_pending_negative_punctuation_token.json
• Updated tests/test_fixtures.py to execute the new fixture categories while preserving existing step/transcript semantics.
• Updated tests/fixtures/README.md to clarify portable serialization/checkpoint fixture contract scope vs Python-local property/fuzz tests.
• Bumped package version to 0.6.18 in pyproject.toml and uv.lock.

Why

• TS now consumes portable conformance artifacts, but several high-risk deterministic core behaviors were still covered only in Python-local tests.
• Promoting serialization and checkpoint edge behaviors into portable fixtures improves cross-port parity coverage and reduces drift risk for 0.6.x behavior.
• Version bump prepares this change set for release.

Checklist

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