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/doc/src/appearance.rs

@@ -162,12 +162,12 @@ impl Document {
     pub fn face_colors(&self, b: &crate::ModelBody) -> Vec<(usize, Look)> {
         // Face colours the body was imported with, under the design's own.
         let to8 = |x: f32| (x.clamp(0.0, 1.0) * 255.0).round() as u8;
-        let names = crate::naming::face_names(b);
         let mut out: Vec<(usize, Look)> = match b.body.paint() {
             Some(p) => p.faces.iter().map(|f| (f.face, Look::custom(f.color.map(to8), f.opacity as f64))).collect(),
-            // Carried from an earlier version of the body: by face name.
+            // Carried from an earlier version of the body: by face name (worked out only here:
+            // the viewport asks every body for its face colours on every change).
             None => match &b.imported {
-                Some(imp) => names
+                Some(imp) => crate::naming::face_names(b)
                     .iter()
                     .enumerate()
                     .filter_map(|(i, n)| imp.faces.get(crate::naming::strip_piece(n)).map(|(c, o)| (i, Look::custom(c.map(to8), *o as f64))))