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.js

@@ -0,0 +1,112 @@
+/* Click-to-zoom lightbox for docs diagrams. Dependency-free.
+ *
+ * Sphinx renders graphviz SVG as <object>, which swallows clicks and
+ * can't be lightboxed. We replace each with an <img> (crisp vector,
+ * clickable), then a click opens a fullscreen overlay with
+ * scroll-to-zoom + drag-to-pan so dense diagrams can be read in detail.
+ * Esc / double-click / background-click closes. Degrades gracefully:
+ * if JS is off, the inline image still renders. */
+(function () {
+  function ready(fn) {
+    if (document.readyState !== "loading") fn();
+    else document.addEventListener("DOMContentLoaded", fn);
+  }
+
+  ready(function () {
+    // 1. graphviz <object> SVG -> clickable <img>
+    document
+      .querySelectorAll('.graphviz object[type="image/svg+xml"]')
+      .forEach(function (obj) {
+        var img = document.createElement("img");
+        img.src = obj.getAttribute("data");
+        img.className = "zoomable";
+        img.alt = "diagram — click to zoom";
+        obj.parentNode.replaceChild(img, obj);
+      });
+
+    // 2. overlay scaffold
+    var ov = document.createElement("div");
+    ov.id = "arb-zoom-overlay";
+    var big = document.createElement("img");
+    ov.appendChild(big);
+    var hint = document.createElement("div");
+    hint.id = "arb-zoom-hint";
+    hint.textContent = "scroll = zoom · drag = pan · Esc / dbl-click = close";
+    document.body.appendChild(ov);
+    document.body.appendChild(hint);
+
+    var scale = 1, tx = 0, ty = 0;
+    var dragging = false, moved = false, sx = 0, sy = 0;
+
+    function apply() {
+      big.style.transform =
+        "translate(" + tx + "px," + ty + "px) scale(" + scale + ")";
+    }
+    function open(src) {
+      ov.classList.add("open");
+      big.onload = function () {
+        var s =
+          Math.min(
+            window.innerWidth / big.naturalWidth,
+            window.innerHeight / big.naturalHeight
+          ) * 0.95;
+        scale = s > 0 ? s : 1;
+        tx = (window.innerWidth - big.naturalWidth * scale) / 2;
+        ty = (window.innerHeight - big.naturalHeight * scale) / 2;
+        apply();
+      };
+      big.src = src;
+    }
+    function close() {
+      ov.classList.remove("open");
+      big.src = "";
+    }
+
+    document.body.addEventListener("click", function (e) {
+      var t = e.target;
+      if (t && t.classList && t.classList.contains("zoomable")) open(t.src);
+    });
+
+    ov.addEventListener(
+      "wheel",
+      function (e) {
+        e.preventDefault();
+        var f = e.deltaY < 0 ? 1.15 : 1 / 1.15;
+        // zoom toward the cursor
+        tx = e.clientX - (e.clientX - tx) * f;
+        ty = e.clientY - (e.clientY - ty) * f;
+        scale *= f;
+        apply();
+      },
+      { passive: false }
+    );
+
+    ov.addEventListener("mousedown", function (e) {
+      dragging = true;
+      moved = false;
+      sx = e.clientX - tx;
+      sy = e.clientY - ty;
+      ov.classList.add("grabbing");
+    });
+    window.addEventListener("mousemove", function (e) {
+      if (!dragging) return;
+      moved = true;
+      tx = e.clientX - sx;
+      ty = e.clientY - sy;
+      apply();
+    });
+    window.addEventListener("mouseup", function () {
+      dragging = false;
+      ov.classList.remove("grabbing");
+    });
+
+    // background click (no drag) closes; dbl-click + Esc close
+    ov.addEventListener("click", function (e) {
+      if (e.target === ov && !moved) close();
+    });
+    ov.addEventListener("dblclick", close);
+    document.addEventListener("keydown", function (e) {
+      if (e.key === "Escape") close();
+    });
+  });
+})();