fix: keep sibling pages whose titles sanitise to the same filename

[blaipr]

Problem

safe_filename() is lossy: Setup: A and Setup A both become Setup_A, Q&A and QA both become QA. In download --recursive, each child was written to sub_dir / f"{safe_filename(title)}.md" with no check for a sibling already at that path, so the second page silently overwrote the first — the tree on disk was missing a page and nothing said so.

Change

A new unique_filename(stem, used) helper gives the first sibling the plain stem and later colliding ones a numeric suffix (Setup_A_2, Setup_A_3, ...), comparing case-insensitively because the common desktop filesystems do. The child loop in download_page tracks the stems used per folder and routes each name through it. The single-page download path is untouched.

Compatibility

Download only. Trees without colliding titles are written exactly as before. upload --recursive takes the page title from the file's # Heading (and the target page from its version marker), so a suffixed filename uploads to the right page unchanged.

Checklist

• ruff check src tests, ruff format --check src tests and python -m pytest tests/ pass
• Tests added for the change (tests/test_recursive_download.py)
• Docs updated (docs/usage.md, download section)
• docs/conversion.md — not needed, no equivalence changed
• CHANGELOG.md updated under Unreleased