engine: mesh.size.* commands, honoured by every mesher (#435)

mesh.size.set adds a size control on faces, bodies, a box or a sphere;
mesh.size.growth sets the growth rate; mesh.size.clear and
mesh.size.list. The tet mesher bakes the controls into its graded field,
the hex meshers, cells and the octree take the graded RegionSize, and
the block projection and mesh plans divide their edges by it.

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

assets/icons/mesh.size.set.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.size.set</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 3 L21 3 L21 21 L3 21 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M12 3 L12 21 M3 12 L21 12" fill="none" stroke="#dde2ea" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M12 12 L21 12 L21 21 L12 21 Z" fill="none" stroke="#4f9ce8" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M16.5 12 L16.5 21 M12 16.5 L21 16.5" fill="none" stroke="#4f9ce8" stroke-linecap="round" stroke-linejoin="round"/>
+  </g>
+</svg>

assets/icons/src/mesh.size.set.json

@@ -0,0 +1 @@
+{"shapes": [{"rect": [3, 3, 18, 18], "fill": "soft", "stroke": "line"}, {"d": "M12 3 L12 21 M3 12 L21 12", "stroke": "line", "width": 1}, {"rect": [12, 12, 9, 9], "fill": null, "stroke": "accent", "width": 1.5}, {"d": "M16.5 12 L16.5 21 M12 16.5 L21 16.5", "stroke": "accent", "width": 1}]}