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/mod.rs

@@ -162,6 +162,7 @@ pub static ALL: &[CommandSpec] = &[
     results::ANIMATE,
     results::FRAME,
     results::REACTIONS,
+    results::EXPORT_CSV,
     results::CLEAR,
     diagnostics::CRASH_REPORTS,
     diagnostics::CRASH_REPORT,