Add ability to split a run into two runs at a character position.

API Design

• run.split(position) — splits the run at the given character index
• Returns tuple (left_run, right_run)
• Both runs inherit the original run's formatting
• Essential for search/replace (Phase D.10: Search and replace with formatting preservation #23) and comment anchoring

Implementation

• Create a new w:r element with same w:rPr
• Move text after position to the new run
• Insert the new run after the original in the paragraph

Upstream: python-openxml#519 (8 comments)