engine, ui: mesh.node.relax and relaxation fields, Relax Nodes tool with sphere and preview (#398)

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/mesh.node.relax.svg

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="24" height="24" viewBox="0 0 24 24">
+  <title>mesh.node.relax</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 12 C3 7.029 7.029 3 12 3 C16.971 3 21 7.029 21 12 C21 16.971 16.971 21 12 21 C7.029 21 3 16.971 3 12 Z" fill="none" stroke="#dde2ea" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round" stroke-dasharray="2 2"/>
+    <path d="M5 12 L19 12 M12 5 L12 19" fill="none" stroke="#dde2ea" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M7.5 8 L16.5 8 M7.5 16 L16.5 16 M8 7.5 L8 16.5 M16 7.5 L16 16.5" fill="none" stroke="#dde2ea" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M9.6 12 C9.6 10.675 10.675 9.6 12 9.6 C13.325 9.6 14.4 10.675 14.4 12 C14.4 13.325 13.325 14.4 12 14.4 C10.675 14.4 9.6 13.325 9.6 12 Z" fill="#4f9ce8" stroke="#4f9ce8" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+  </g>
+</svg>

assets/icons/src/mesh.node.relax.json

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+{"shapes": [{"circle": [12, 12, 9], "stroke": "line", "width": 1.25, "dash": [2, 2]}, {"d": "M5 12 L19 12 M12 5 L12 19", "stroke": "line", "width": 1.0}, {"d": "M7.5 8 L16.5 8 M7.5 16 L16.5 16 M8 7.5 L8 16.5 M16 7.5 L16 16.5", "stroke": "line", "width": 1.0}, {"circle": [12, 12, 2.4], "fill": "accent", "stroke": "accent"}]}