test: add controller conformance fixtures

[rlippmann]

What changed

• Added a new shared controller conformance fixture suite under tests/fixtures/conformance/controller/.
• Added portable fixture coverage for:
  • step(engine, user_input) result envelope and state snapshot
  • preview(engine, user_input) mutating and non-mutating paths
  • preview clarify path and pending-state consistency
  • state_diff(state_before, state_after) structural diff output
• Extended tests/test_fixtures.py with a new conformance runner for controller fixtures.
• Documented the controller fixture suite in tests/fixtures/README.md.

Why

• TypeScript 0.7 needs portable shared controller contracts, rather than relying on Python-only unit tests.
• These fixtures provide deterministic, language-neutral expectations for controller APIs without changing semantics.

Checklist

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