index: card for "One sphere, four turns" (the base case), first among the explorers

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FAqjv3n4yABxvJPJHFRhVA

index.html

@@ -64,6 +64,11 @@ a.card p{margin:0;color:var(--muted);font-size:12.5px}
   <video controls playsinline preload="none" src="https://teahub.xyz/maths/videos/raw/commit/c8727a099a27a67e70823ba3bdc50378c95f00f5/lamp.mp4"></video>
   <p class="lede">The same ball in two explorers: put ½ at the centre and the critical line becomes the strut through 0 and ∞, the functional equation a sign flip through the centre. Swap ζ for a function with the same symmetry and no Euler product (Davenport–Heilbronn) and the zeros step off the strut: symmetry alone does not pin them, the arithmetic is the extra incidence.</p>
   <div class="cards">
+    <a class="card" href="turns/">
+      <h2>One sphere, four turns</h2>
+      <p>The base case. ζ painted on the Riemann sphere, 0 at the bottom, ∞ at the top, and 0, 1, ∞, −1 a quarter turn apart — 1 is half way to ∞. Three sliders turn it about its three struts; the tip about ±i has four stops whose shadows are the charts s/(1 − s), 2s − 1 (zeta’s orientation), 1 − 1/s (Li) and 1/(1 − 2s). The ball beside its shadow, with the six frame points and the zeros as beads.</p>
+      <span class="go">open explorer →</span>
+    </a>
     <a class="card" href="strut/">
       <h2>The zeta strut</h2>
       <p>ζ, ξ and Davenport–Heilbronn painted in the s-plane, in w = 1/(s − ½), and on the Riemann sphere with ½ in front; Hardy's Z(t) along the strut, with the off-line zero at t ≈ 85.7 marked.</p>