refactor(inspect): centralize text-frame XML access via helper

[tmustier]

Summary

Continue the dynamic-access cleanup by routing remaining text-frame private XML probing in inspect_pptx.py through shared pptx_access helpers.

Changes

New helper

• clean_slides/pptx_access.py
  • added text_frame_xml_element(...) to centralize access to text-frame _element.

Inspect migration

• clean_slides/inspect_pptx.py
  • _parse_text_frame(...) now uses text_frame_xml_element(...) instead of direct tf._element access.
  • inspect_layout(...) paragraph default-style inspection now uses paragraph_xml_element(...) and guarded cast flow.
  • _paragraph_xml(...) now returns a casted XmlElement after helper guard.
  • RunInfo.to_dict(...) no longer uses reflective getattr; switched to explicit ordered field emission.

Tests

• tests/test_pptx_access.py
  • added coverage for text_frame_xml_element(...).

Validation

• .venv/bin/ruff check clean_slides tests
• .venv/bin/pyright
• .venv/bin/pytest -q
• .venv/bin/pre-commit run --all-files

All green locally.