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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -182,6 +182,7 @@ pub static ALL: &[CommandSpec] = &[
     results::FRAME,
     results::REACTIONS,
     results::EXPORT_CSV,
+    results::EXPORT_VTU,
     results::CLEAR,
     diagnostics::CRASH_REPORTS,
     diagnostics::CRASH_REPORT,