front page: the four-turns explorer and the zeta ball are one card now

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

index.html

@@ -64,9 +64,9 @@ 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>
+    <a class="card" href="zeta/">
+      <h2>Zeta ball · four turns</h2>
+      <p>One sphere, dragged by hand or turned by its three struts. Stand it up for the base case — 0 at the floor, ∞ at the lamp, and 0, 1, ∞, −1 a quarter turn apart, so 1 is half way to ∞ — then tip it through four stops whose shadows are the charts s/(1 − s), 2s − 1 (zeta’s orientation), 1 − 1/s (Li) and 1/(1 − 2s). The slide runs the touching point round the critical circle, and dragging the ball moves the sliders. Live ζ or ξ on the ball and on the floor, the six frame points under both names, the zeros as beads, and 3b1b’s grid as the ball’s own latitude–longitude, bent by η.</p>
       <span class="go">open explorer →</span>
     </a>
     <a class="card" href="strut/">
@@ -89,11 +89,6 @@ a.card p{margin:0;color:var(--muted);font-size:12.5px}
       <p>The ball projected from the other pole: lamp at the touch point ½, screen at ∞, u = ¼/(s − ½). Same frame, the functional equation a point reflection, the dial a plain rotation. Every zero falls into the centre — trivial along the mirror from one side, critical along the strut from both — and the critical strip becomes the outside of two kissing circles. Paint ζ, ξ, or χ = ζ(s)/ζ(1 − s), the gamma factor on its own.</p>
       <span class="go">open explorer →</span>
     </a>
-    <a class="card" href="zeta/">
-      <h2>Zeta ball</h2>
-      <p>The earlier picture: ζ on a ball of diameter ½ resting at ½, turned by quaternion quarter-turns; 3b1b's grid as the ball's own latitude–longitude, bent by η.</p>
-      <span class="go">open explorer →</span>
-    </a>
   </div>
   <section class="notes">
     <p><span class="dot" style="background:var(--up)"></span>four upward triangles · <span class="dot" style="background:var(--down)"></span>five downward · <span class="dot" style="background:var(--bindu)"></span>the bindu</p>