engine: results.export.vtu writes a solved step for ParaView

Static, frequency (per mode), nonlinear and explicit (per frame, with a
.pvd collection) and heat results; fluid steps are refused. Wedges keep
the model's node order: VTK 9's wedge has the same orientation.

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

assets/icons/results.export.vtu.svg

@@ -0,0 +1,10 @@
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+  <title>results.export.vtu</title>
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+  </g>
+</svg>

assets/icons/src/results.export.vtu.json

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