Graphviz renders SVG as <object>, which swallows clicks and can't be
lightboxed. Vendored, dependency-free zoom.js converts each graphviz
<object> to a clickable <img> (crisp vector) and adds a fullscreen
overlay: click to open, scroll to zoom toward the cursor, drag to pan,
Esc / dbl-click / background-click to close. Works for any img.zoomable,
so future charts get it free. Degrades gracefully (no JS -> inline image
still renders). Wired via html_css_files / html_js_files.
