Phase 7 (#1075): Lock down state.py

[xbrianh]

Part of #1075. Depends on #1094 (Phase 5) and #1095 (Phase 6) both merged.

What

Once no external file imports from gremlins.executor.state:

1. Rename State → _State and StateData → _StateData (underscore = internal).
2. Set __all__ = [] or remove the module's public names.
3. Add a module-level comment at the top of state.py:

   # Internal to gremlins.executor — import through Gremlin, not directly.

4. Add a CI lint rule (or grep check) to prevent regressions:

   grep -r "from gremlins.executor.state" .
   

Files

• gremlins/executor/state.py
• CI/lint config