Fixes: - CRITICAL: Task 2 Step 3 no longer overwrites md2doc.py — extends in place - MAJOR: COMMON.keep_with_next_headings now wired into _render_heading - MINOR: added test for keep_with_next assertion - MINOR: Task 9 note about footer param preservation - MINOR: _render_pre now preserves newlines via add_break() - MINOR: Notes section documents fixture location, security, COMMON wiring