Landing: add the flow

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

index.html

@@ -63,6 +63,11 @@ a.card p{margin:0;color:var(--muted);font-size:12.5px}
       <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>
       <span class="go">open explorer →</span>
     </a>
+    <a class="card" href="flow/">
+      <h2>The zeta flow</h2>
+      <p>Sphere → zeta picture → the full step, as one Möbius flow z ↦ z/(tz + 1). At every t the ball is placed in space (tipped 2·arctan t, radius 1/(1 + t²), resting at t/(1 + t²)) and its shadow follows. The equator is straight only at t = 1; the free pole walks 1, ½, ⅓, …</p>
+      <span class="go">open explorer →</span>
+    </a>
     <a class="card" href="dial/">
       <h2>The zeta dial</h2>
       <p>Turn the ball about the ½–∞ strut: s − ½ ↦ e<sup>iθ</sup>(s − ½). A quarter turn is the Wick rotation (cosh onto cos), a half turn is s ↦ 1 − s, a full turn is q = −1. Three sheets: ξ (even), (s − ½)·ξ (odd, with a zero at ½), and F(u) with u = (s − ½)², turning at double speed.</p>