engine: result units, per-step staleness, step symbols, true-scale nonlinear results

- Quantity names the unit of every number the results show from the model's
  unit system (mm, MPa, N, N·mm, mJ, mW, °C or K); Field::title puts it in the
  legend title, and calls a mode shape normalised (#321).
- A solved step's result stays current through edits that leave what it reads
  alone (Model::step_inputs: other steps and what only they use), and comes
  back current on undo (#318).
- Session::step_glyphs: the symbols of what a solved step used, for drawing
  over its results; no load arrows on a frequency step's modes (#327).
- Nonlinear static steps open deformed at true scale like explicit ones, and
  their automatic scale is 1 (auto_scale, shared by the viewport and the
  headless pictures) (#326).
- results.reactions with an empty sets list sums the step's boundary-condition
  sets, as its form says (#328).

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

crates/engine/src/cmd/solve.rs

@@ -1397,8 +1397,8 @@ pub(crate) mod tests {
         s.execute("model.step.create", &json!({"name": "Long", "procedure": long})).unwrap();
         assert!(s.execute("solve.explicit", &json!({"step": "Long"})).unwrap_err().to_string().contains("increments"));
         assert!(s.execute("model.set.delete", &json!({"name": "all_nodes"})).unwrap_err().to_string().contains("step `Impact`"));
-        // Defining steps edited the model, so the impact result is stale.
-        assert_eq!(s.solved_steps().count(), 0);
+        // Defining other steps left what the impact step reads alone: its result is current.
+        assert_eq!(s.solved_steps().collect::<Vec<_>>(), ["Impact"]);
     }
 
     /// A mass on a spring and a dashpot, defined, solved and written through commands; editing