site(learn): active section highlight + other-essays switcher on left rail

- Added scroll-spy to the chapter rail: as you scroll, the H2 closest
  to the upper third of the viewport becomes the active link (white
  background, sky text, sky ring).
- Below the chapter rail, an 'Other essays' list shows every other
  learn article (6 on each page) with read-time, plus an 'All articles'
  link back to the library.

site/src/layouts/Learn.astro

@@ -53,11 +53,37 @@ const next = related.filter((r) => r.href !== path).slice(0, 2);
 
     <!-- LEFT RAIL -->
     <aside class="hidden lg:block border-r border-[color:var(--border)] bg-[color:var(--surface-1)]/40">
-      <div class="sticky top-0 max-h-screen overflow-y-auto py-8 px-6">
-        <div class="text-[10.5px] uppercase tracking-[0.22em] font-mono text-muted-foreground mb-4">On this page</div>
-        <ol class="learn-chapter-rail space-y-px text-[13px]" data-chapter-rail>
-          <li class="text-[11.5px] text-muted-foreground/60 italic">Loading…</li>
-        </ol>
+      <div class="sticky top-0 max-h-screen overflow-y-auto py-8 px-6 flex flex-col gap-7">
+        <!-- On this page -->
+        <div>
+          <div class="text-[10.5px] uppercase tracking-[0.22em] font-mono text-muted-foreground mb-4">On this page</div>
+          <ol class="learn-chapter-rail space-y-px text-[13px]" data-chapter-rail>
+            <li class="text-[11.5px] text-muted-foreground/60 italic">Loading…</li>
+          </ol>
+        </div>
+
+        <!-- Other articles in the library -->
+        <div class="pt-6 border-t border-[color:var(--border)]">
+          <div class="text-[10.5px] uppercase tracking-[0.22em] font-mono text-muted-foreground mb-4 flex items-center justify-between">
+            <span>Other essays</span>
+            <span class="font-mono text-[10.5px] text-muted-foreground/70">{related.length - 1}</span>
+          </div>
+          <ol class="space-y-px text-[13px]">
+            {related.filter((r) => r.href !== path).map((r, i) => (
+              <li>
+                <a href={r.href} class="group flex items-baseline gap-3 py-1.5 px-2 -mx-2 rounded-md text-foreground/65 hover:bg-white hover:text-[#0369a1] transition-colors">
+                  <span class="font-mono text-[10.5px] text-muted-foreground/60 w-5 shrink-0">{String(i + 1).padStart(2, '0')}</span>
+                  <span class="leading-snug flex-1 truncate">{r.title}</span>
+                  <span class="font-mono text-[10px] text-muted-foreground/55 shrink-0">{r.read.replace(' read', '').replace(' min', 'm')}</span>
+                </a>
+              </li>
+            ))}
+          </ol>
+          <a href="/learn/" class="mt-4 inline-flex items-center gap-1.5 text-[11.5px] font-mono text-[#0369a1] hover:text-[#075985]">
+            All articles
+            <Icon name="arrowRight" size={10} />
+          </a>
+        </div>
       </div>
     </aside>
 
@@ -113,26 +139,71 @@ const next = related.filter((r) => r.href !== path).slice(0, 2);
   </div>
 
   <script is:inline>
-    // Auto-build chapter rail from H2 elements in the prose body
+    // Auto-build chapter rail from H2 elements + scroll-spy active state
     (function () {
       const rail = document.querySelector('[data-chapter-rail]');
       const main = document.querySelector('.learn-prose');
       if (!rail || !main) return;
       const slug = (s) => s.toLowerCase().replace(/[^a-z0-9]+/g, '-').replace(/^-|-$/g, '');
-      const h2s = main.querySelectorAll('h2');
+      const h2s = Array.from(main.querySelectorAll('h2'));
       if (!h2s.length) return;
 
+      const baseClass    = 'group flex items-baseline gap-3 py-1.5 px-2 -mx-2 rounded-md transition-colors';
+      const inactiveClass = ' text-foreground/70 hover:bg-white hover:text-[#0369a1]';
+      const activeClass   = ' bg-white text-[#0369a1] font-semibold ring-1 ring-[color:var(--sky-2)]';
+
+      // Build rail
       rail.innerHTML = '';
+      const links = [];
       h2s.forEach((h2, i) => {
-        if (!h2.id) h2.id = slug(h2.textContent || `section-${i + 1}`);
+        if (!h2.id) h2.id = slug(h2.textContent || 'section-' + (i + 1));
         const li = document.createElement('li');
         const a  = document.createElement('a');
-        a.href   = `#${h2.id}`;
-        a.className = 'group flex items-baseline gap-3 py-1.5 px-2 -mx-2 rounded-md text-foreground/70 hover:bg-white hover:text-[#0369a1] transition-colors';
-        a.innerHTML = `<span class="font-mono text-[10.5px] text-muted-foreground/70 w-5 shrink-0">${String(i + 1).padStart(2, '0')}</span><span class="leading-snug">${h2.textContent}</span>`;
+        a.href   = '#' + h2.id;
+        a.className = baseClass + inactiveClass;
+        a.innerHTML = '<span class="font-mono text-[10.5px] text-muted-foreground/70 w-5 shrink-0 chapter-num">' + String(i + 1).padStart(2, '0') + '</span><span class="leading-snug">' + h2.textContent + '</span>';
         li.appendChild(a);
         rail.appendChild(li);
+        links.push({ id: h2.id, el: a });
       });
+
+      // Scroll-spy: highlight closest H2 above viewport mid-line
+      let currentId = null;
+      function setActive(id) {
+        if (id === currentId) return;
+        currentId = id;
+        links.forEach((l) => {
+          if (l.id === id) {
+            l.el.className = baseClass + activeClass;
+          } else {
+            l.el.className = baseClass + inactiveClass;
+          }
+        });
+      }
+
+      function onScroll() {
+        const probe = window.innerHeight * 0.35;
+        let active = h2s[0].id;
+        for (const h2 of h2s) {
+          const rect = h2.getBoundingClientRect();
+          if (rect.top - probe <= 0) {
+            active = h2.id;
+          } else {
+            break;
+          }
+        }
+        setActive(active);
+      }
+
+      onScroll();
+      let raf = 0;
+      window.addEventListener('scroll', () => {
+        if (raf) return;
+        raf = requestAnimationFrame(() => {
+          raf = 0;
+          onScroll();
+        });
+      }, { passive: true });
     })();
   </script>
 </Layout>