Rebuild stack-folder with sticky tab rail and site previews.

L0–L7 folders stack on scroll with aligned max depth, labeled tabs that
stay consistent when L7 joins the rail, Cal embeds, preview screenshots,
Playwright tests, and updated README.

Co-authored-by: Cursor <cursoragent@cursor.com>

stack-folder/folder-rail.js

@@ -0,0 +1,32 @@
+/** Fixed rail — all L0–L7 visible; click to jump (syncs with scroll) */
+export function initRailRoller(rail = document.querySelector('.tab-rail')) {
+  const tabs = rail ? [...rail.querySelectorAll('.rail-tab')] : [];
+  if (!rail || !tabs.length) return;
+
+  function setActive(layer) {
+    const n = Math.max(0, Math.min(tabs.length - 1, layer));
+    tabs.forEach((btn, i) => btn.classList.toggle('active', i === n));
+  }
+
+  function gotoLayer(layer) {
+    setActive(layer);
+    window.dispatchEvent(
+      new CustomEvent('stack-goto-layer', { detail: { layer } }),
+    );
+  }
+
+  tabs.forEach((btn) => {
+    btn.addEventListener('click', (e) => {
+      e.preventDefault();
+      e.stopPropagation();
+      gotoLayer(Number(btn.dataset.layer ?? btn.dataset.goto));
+    });
+  });
+
+  window.addEventListener('stack-depth', (e) => {
+    const layer = Number(e.detail?.active);
+    if (Number.isFinite(layer)) setActive(layer);
+  });
+
+  setActive(0);
+}