results: export probe and reaction results to CSV (results.export.csv, #92)

results.export.csv runs solve.probe or results.reactions and writes their reply as an RFC 4180
CSV: a row per node (node, x, y, z and the quantities the step has, columns it does not have
left out) or per set with the total and the applied load; numbers in shortest round-trip form,
so they read back bit for bit. A stale result is refused. The results panel has Export CSV…
beside Reaction sums and under the probed node: the save picker, or the form without one.

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

crates/engine/src/cmd/forms.rs

@@ -1687,6 +1687,13 @@ pub const FRAME: &[Field] = &[
     int("frame", "Frame", 0, 1_000_000_000).help("The frame shown."),
     positive("time", "Time", D::Time).help("Show the frame nearest this time; the last when empty.").hint("the last frame"),
 ];
+pub const EXPORT_CSV: &[Field] = &[
+    choice("what", "Export", &[("reactions", "Reaction sums"), ("probe", "Probed nodes")]).help("What to write.").req().def("\"reactions\""),
+    path("path", "File", true).help("The CSV file to write.").req(),
+    SOLVED,
+    references("sets", "Sets", C::Set).help("Reactions: the sets to sum; the step's BC sets when empty.").hint("the step's BC sets"),
+    json("nodes", "Nodes").help("Probe: the node labels, e.g. [1, 2, 3]."),
+];
 pub const REACTIONS: &[Field] = &[
     SOLVED,
     references("sets", "Sets", C::Set)