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/lib.rs

@@ -49,9 +49,10 @@ pub use assembly::{
 };
 pub use document::*;
 pub use eval::{
-    EdgeRef, FeatureResult, Model, ModelBody, ModelState, RevolveExtent, SolvedSketch, ThreadInfo, curve_pick_error, edge_names_for, edge_refs,
-    face_point_names, face_point_state, hole_direction_at, object_height, parse_metric_thread, pattern_matrices, revolve_extent,
+    EdgeRef, FeatureResult, Model, ModelBody, ModelState, PlacedBodies, RevolveExtent, SolvedSketch, ThreadInfo, curve_pick_error, edge_names_for,
+    edge_refs, face_point_names, face_point_state, hole_direction_at, object_height, parse_metric_thread, pattern_matrices, revolve_extent,
     revolve_extent_with_convention, select_profiles, sketch_hole_direction, state_in_frame, state_in_frame_at, through_all_distance, world_state,
+    world_state_reusing,
 };
 pub use params::{ParamRow, ParamUser, cycles};
 pub use solidcraft_kernel as kernel;