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

docs/features.md

@@ -142,7 +142,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Display | Mode Shapes — Mode shapes and frequencies of a frequency step | ready | `results.modes` |
 | Display | Animate — Animate the deformed shape, a mode or an explicit step's frames | ready | `results.animate`, `results.frame` |
 | Query | Probe — Displacement, rotation, reaction and stress at nodes, element section forces, a mode shape, or an explicit field frame (or click the model) | ready | `solve.probe`, `solve.history` |
-| Query | Reactions — Reaction force and moment sums per set | ready | `results.reactions` |
+| Query | Reactions — Reaction force and moment sums per set | ready | `results.reactions`, `results.export.csv` |
 | Query | Free Body — Free-body forces across a cut | coming soon (M9) |  |
 | Plots | XY Plot — Plot a quantity against time or another | coming soon (M9) |  |
 | Plots | Path Plot — Plot a quantity along a path | coming soon (M9) |  |