engine: geometry.faces.merge, faces meshed as one (virtual topology, #505)

geometry.faces.merge {name, faces} records a face merge in the model;
it is applied after the partitions whenever the design is opened or cut
again, undoes through the journal and is saved with the model. Meshers,
mesh.plan (features and rings see merge:<name>; a merged face is
prismatic when its B-rep faces are), checks and the viewport see one
face, drawn in the warm colour. geometry.faces.merge.delete removes it.

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

assets/icons/geometry.faces.merge.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>geometry.faces.merge</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 8 L12 4 L21 8 L12 12 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M3 8 L3 16 L12 20 L12 12 M12 20 L21 16 L21 8" fill="none" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M7.5 6 L16.5 10" fill="none" stroke="#f0b44a" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round" stroke-dasharray="1.5 2"/>
+    <path d="M3 8 L12 4 L21 8 L12 12 Z" fill="none" stroke="#4f9ce8" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"/>
+  </g>
+</svg>

assets/icons/src/geometry.faces.merge.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 8 L12 4 L21 8 L12 12 Z", "fill": "soft", "stroke": "line"}, {"d": "M3 8 L3 16 L12 20 L12 12 M12 20 L21 16 L21 8", "stroke": "line"}, {"line": [7.5, 6, 16.5, 10], "stroke": "warm", "width": 1.25, "dash": [1.5, 2]}, {"d": "M3 8 L12 4 L21 8 L12 12 Z", "stroke": "accent", "width": 1.75}]}