feat: add controller result helpers

[rlippmann]

What changed

• added a minimal public helper set for controller result objects:
  • get_step_decision
  • get_step_state
  • get_preview_decision
  • get_preview_state_after
  • preview_would_mutate
  • diff_has_changes
• exported the new helpers from context_compiler and added controller tests for helper behavior and importability
• updated the README controller quick example and examples/08_controller_preview_diff.py to use the new helpers instead of teaching direct dict traversal for common reads
• updated REPL/controller rendering internals to use the new controller accessors where that improved consistency without changing output
• updated the conformance/public-API fixture docs so controller helper parity is an explicit cross-language target while raw controller envelope fixtures remain unchanged

Why

• controller result envelopes are already public and stable, but the public API direction elsewhere prefers helpers for inspecting decisions and state
• these changes make controller usage more consistent with that helper-first style while keeping the existing result dict shapes fully valid
• the conformance docs update makes the new helper surface explicit for ports without weakening the existing raw-envelope controller contract

Checklist

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