docs: click-to-zoom lightbox for diagrams (scroll-zoom + drag-pan)

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.

docs/_source/_static/zoom.css

@@ -0,0 +1,43 @@
+/* Click-to-zoom for diagrams (graphviz SVG + any img.zoomable).
+   Vendored, dependency-free. See zoom.js. */
+
+.graphviz img,
+img.zoomable {
+  cursor: zoom-in;
+  max-width: 100%;
+}
+
+#arb-zoom-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 100000;
+  background: rgba(10, 12, 16, 0.92);
+  display: none;
+  overflow: hidden;
+  cursor: grab;
+}
+#arb-zoom-overlay.open { display: block; }
+#arb-zoom-overlay.grabbing { cursor: grabbing; }
+
+#arb-zoom-overlay img {
+  position: absolute;
+  top: 0;
+  left: 0;
+  max-width: none;
+  transform-origin: 0 0;
+  user-select: none;
+  -webkit-user-drag: none;
+}
+
+#arb-zoom-hint {
+  position: fixed;
+  top: 10px;
+  right: 16px;
+  z-index: 100001;
+  display: none;
+  color: #fff;
+  font: 13px/1.4 system-ui, sans-serif;
+  opacity: 0.75;
+  pointer-events: none;
+}
+#arb-zoom-overlay.open ~ #arb-zoom-hint { display: block; }