refactor: make engine state opaque

[rlippmann]

What changed

• Removed the public raw-state surface from the engine:
  • removed exported State
  • removed Engine.state
  • removed state= from Engine.__init__
  • removed state= from create_engine()
• Migrated supported consumers to semantic engine APIs:
  • engine.premise
  • engine.policies
  • export_json() / import_json()
• Updated REPL, demos, examples, evals, tests, fixtures, and documentation to stop depending on public raw state.
• Replaced state-based test setup and assertions with supported public behavior and persistence boundaries.
• Removed obsolete mutation-isolation fixtures for constructor state injection and public state snapshots.
• Updated fixture documentation to reflect the new public surface.

Why

• The public raw state representation exposed internal implementation details and increased cross-language API maintenance.
• Consumers generally need semantic state access or persistence, not the engine's internal state structure.
• Keeping authoritative state private allows the engine implementation to evolve while preserving a smaller, clearer public contract.

Checklist

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