Enhance type safety and simplify subprocess execution

[allixender]

Improve the developer experience and internal reliability:

• Define TypedDict or dataclass return types for complex API methods (e.g., dgapi_grid_gen) to provide better IDE support and static analysis.
• Refactor the manual subprocess.Popen polling loop in DGGRID.run() to use subprocess.run where real-time streaming is not required, making the logic more concise and error-resistant.
• Address potential circular dependencies or confusing imports in experimental modules like igeo7_ext.py.