Add document-wide search and replace that preserves run formatting. Upstream #30 (39 comments).

API Design

• document.search(text) — returns list of SearchMatch objects
• document.replace(old_text, new_text) — replaces all occurrences, preserving the formatting of the first character's run
• SearchMatch: .paragraph, .paragraph_index, .run_indices, .start, .end
• Options: case_sensitive (default True), whole_word (default False)

Challenges

• Text may span multiple runs (e.g., "hel" in one run, "lo" in another)
• Replacement must merge/split runs appropriately
• Must preserve bold/italic/font of the original runs

Implementation

• Walk all paragraphs, reconstruct full text from runs with character-to-run mapping
• Find matches in the full text
• Map match positions back to runs
• Replace text in the affected runs, splitting if needed

Upstream issue: #30