Support documents containing tracked changes (pandoc --track-changes)

[Tishinator]

Problem

Documents with unresolved tracked changes convert using pandoc's default (--track-changes=accept) implicitly; we neither expose control over it nor warn that revisions were resolved.

Proposed fix

• Pass --track-changes=accept explicitly in src/docx2md/engine.py (deterministic, documented behavior).
• Optionally expose --track-changes {accept,reject} on the CLI.
• Emit a warning when the source document contains revision marks (w:ins/w:del present in document.xml — cheap check in the pre-pass), since silently accepting edits can change meaning (SRS-FR-018 material-loss threshold).

Comments (w:comment) remain out of scope for rendering; consider a count warning only.

Acceptance

• Fixture with tracked insertions/deletions converts to the accepted text with a warning.
• Behavior documented in README.