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/catalogue.rs

@@ -573,7 +573,7 @@ pub static CATALOGUE: &[Capability] = &[
         "Displacement, rotation, reaction and stress at nodes, element section forces, a mode shape, or an explicit field frame (or click the model)",
         &["solve.history"],
     ),
-    ready("results.reactions", "Reactions", Results, "Query", "Reaction force and moment sums per set"),
+    ready_more("results.reactions", "Reactions", Results, "Query", "Reaction force and moment sums per set", &["results.export.csv"]),
     soon("results.free_body", "Free Body", Results, "Query", "M9", "Free-body forces across a cut"),
     soon("results.xy", "XY Plot", Results, "Plots", "M9", "Plot a quantity against time or another"),
     soon("results.path", "Path Plot", Results, "Plots", "M9", "Plot a quantity along a path"),