Phase 4 (#1075): Migrate parallel.py + parallel_state.py

[xbrianh]

Part of #1075. Depends on #1092 (Phase 3).

What

parallel.py imports both State and StateData and constructs StateData() instances directly — more than any other stage file.

• _ParallelExecutor.__init__ currently takes parent_data: StateData. Change to accept parent_state: State (use gremlin.state) and derive the same fields from parent_state.data.*.
• Remove the two StateData() constructor calls in parallel.py.
• Replace all state.X with gremlin.state.X in stage bodies.
• Remove from gremlins.executor.state import State, StateData from parallel.py.
• parallel_state.py imports StateData and resolve_state_file; audit whether those can be replaced by accessing fields through the Gremlin instance that ParallelStage already holds via self.gremlin.

Files

• gremlins/stages/parallel.py
• gremlins/executor/parallel_state.py