engine: geometry.stitch, imported surfaces stitched for meshing (#507)

geometry.stitch {name, surface, tolerance} records a stitch; the
surfaces keep what was read (TaggedSurface::original) and are stitched
again from it whenever a surface is imported, the model opened, or a
stitch added or deleted (geometry.stitch.delete). Opening a model also
warns about merges that no longer apply.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/geometry.stitch.svg

@@ -0,0 +1,11 @@
+<?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.stitch</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 6 L10.5 6 L10.5 18 L3 18 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M13.5 6 L21 6 L21 18 L13.5 18 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M9 8.5 L15 8.5" fill="none" stroke="#f0b44a" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M9 12 L15 12" fill="none" stroke="#f0b44a" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M9 15.5 L15 15.5" fill="none" stroke="#f0b44a" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"/>
+  </g>
+</svg>

assets/icons/src/geometry.stitch.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 6 L10.5 6 L10.5 18 L3 18 Z", "fill": "soft", "stroke": "line"}, {"d": "M13.5 6 L21 6 L21 18 L13.5 18 Z", "fill": "soft", "stroke": "line"}, {"line": [9, 8.5, 15, 8.5], "stroke": "warm", "width": 1.75}, {"line": [9, 12, 15, 12], "stroke": "warm", "width": 1.75}, {"line": [9, 15.5, 15, 15.5], "stroke": "warm", "width": 1.75}]}