Logo 3 (cycle): add a radial indigo glow inside the loop

Places a r=20 circle filled with a radial-gradient (indigo @ 0.55 in
the centre, fading to transparent at the cycle's outer border) behind
the arc and phase dots, so the centre dot reads as a halo rather than
a flat marker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

doc/logo-3-cycle.svg

@@ -3,6 +3,14 @@
     svg:root { color: #0f172a; }
     @media (prefers-color-scheme: dark) { svg:root { color: #e2e8f0; } }
   </style>
+  <defs>
+    <radialGradient id="cycle-glow" cx="32" cy="32" r="20" gradientUnits="userSpaceOnUse">
+      <stop offset="0"   stop-color="#6366f1" stop-opacity="0.55"/>
+      <stop offset="0.6" stop-color="#6366f1" stop-opacity="0.12"/>
+      <stop offset="1"   stop-color="#6366f1" stop-opacity="0"/>
+    </radialGradient>
+  </defs>
+  <circle cx="32" cy="32" r="20" fill="url(#cycle-glow)"/>
   <path d="M52 32 A20 20 0 1 1 32 12" stroke="currentColor" stroke-width="3.5" stroke-linecap="round"/>
   <path d="M44 8 L52 12 L48 20" stroke="currentColor" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
   <circle cx="32" cy="32" r="5" fill="#6366f1"/>

doc/logo.html

@@ -195,6 +195,12 @@
       </symbol>
 
       <symbol id="logo-3" viewBox="0 0 64 64">
+        <radialGradient id="cycle-glow" cx="32" cy="32" r="20" gradientUnits="userSpaceOnUse">
+          <stop offset="0"   stop-color="#6366f1" stop-opacity="0.55"/>
+          <stop offset="0.6" stop-color="#6366f1" stop-opacity="0.12"/>
+          <stop offset="1"   stop-color="#6366f1" stop-opacity="0"/>
+        </radialGradient>
+        <circle cx="32" cy="32" r="20" fill="url(#cycle-glow)"/>
         <path d="M52 32 A20 20 0 1 1 32 12" stroke="currentColor" stroke-width="3.5" stroke-linecap="round" fill="none"/>
         <path d="M44 8 L52 12 L48 20" stroke="currentColor" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
         <circle cx="32" cy="32" r="5" fill="#6366f1"/>