Agent-built designs: stop redoing whole-design work on every command (#1148)

A design built over MCP is many small commands, and each one did work over the
whole design, so building or showing a 1,300-body design took minutes:

- moved components were transformed and meshed again after every change; world_state
  now reuses each placed body whose definition and placement are unchanged
- next_d_name searched the design once per candidate number
- Document::eval evaluated every parameter for every expression; now only when the
  expression names one
- param_rows' used_by was quadratic; the desktop no longer gathers completions on every
  change while no text box has focus

solidcraft-cli bench-agent builds a generated ~1,300-body bicycle command by command
and times it (241 s -> 16 s here). Eval output is unchanged apart from timings.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/params_dialog.rs

@@ -120,7 +120,12 @@ pub fn open(app: &mut SolveApp) {
 
 /// Draw the dialog when it is open. Called once per frame.
 pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
-    publish(app, ctx);
+    // Completions are offered only under a focused text box; gathering them evaluates every
+    // parameter, so a design changing command after command (an agent building over MCP)
+    // does not pay for it on every change while nobody is typing.
+    if app.params.is_some() || ctx.memory(|m| m.focused().is_some()) {
+        publish(app, ctx);
+    }
     let Some(mut d) = app.params.take() else { return };
     let t = Tokens::get();
     let mut open = true;