fix: prefer playwright headless shell for pdf rendering

[developer0hye]

What changed

• Prefer Playwright headless shell installations before normal Chrome/Edge/Chromium when MARKNEST_BROWSER_PATH is not set
• Add browser resolution tests for configured path precedence, headless-shell precedence, and highest-version selection
• Update README browser discovery notes for the new runtime order

Why

• System Chrome could surface a visible window during marknest convert
• The headless-shell binary gives a more predictable headless path for PDF rendering

Testing

• cargo fmt --all
• cargo test -p marknest
• cargo run -p marknest -- convert README.md --render-report target/headless-shell-main-report.json -o target/headless-shell-main.pdf
• Manual GitHub URL conversions against jsvine/pdfplumber, karpathy/autoresearch, and mermaid-js/mermaid

Related: none