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/lib.rs

@@ -545,6 +545,12 @@ mod tests {
             json!({"format": "nastran", "path": "/no/such/dir/part.bdf", "steps": vec!["x".repeat(1000); 2000]}),
             json!({"format": "nastran", "path": "/no/such/dir/part.bdf", "steps": {}}),
             json!({"path": 7}),
+            json!({"path": "/no/such/dir/out.vtu", "step": "Nope"}),
+            json!({"path": "/no/such/dir/out.vtu", "mode": 0}),
+            json!({"path": "/no/such/dir/out.pvd", "mode": 1_000_000_000}),
+            json!({"path": "/no/such/dir/out.pvd", "frames": 3, "encoding": []}),
+            json!({"path": "x\u{0}.vtu"}),
+            json!({"path": format!("/no/such/{}.vtu", "x".repeat(100_000))}),
             json!({"path": "x\u{0}.stl"}),
             json!({"path": "/no/such/dir/skin.stl", "set": 7}),
             json!({"path": "/no/such/dir/skin.stl", "set": "x".repeat(100_000)}),