ui-egui: select faces, edges, vertices, bodies, nodes and elements by picking

A click in the viewport picks what is under the pointer through the
selection filter (the selection bar, top centre): geometry faces, edges,
vertices and bodies, or mesh nodes, element faces and elements. Shift
adds, Cmd/Ctrl adds or removes, a click on nothing clears, Escape
clears; with Box on, a left drag selects the visible nodes, element
faces or elements inside the box. What a click would pick is
highlighted under the pointer with its name beside it.

The selection is kept in the model's persistent terms: geometry by name
(GeometryRef, named through the mesh's geometry tags or the drawn
bodies' face names), so it survives a remesh, and the mesh by label
(labels the model loses drop out). It reads as a set's members.

Picking runs on the CPU against the triangles both renderers draw. The
selection and the hover are drawn as an overlay in the theme's new
pick_selected and pick_hover colours (light and dark), outlined; the GPU
draws it as a second small layer, so a hover change uploads only that.

Command forms that apply to part of the model (sets, boundary
conditions, loads, sections, couplings) get a Target row whose Use
selection fills the members, or makes a set of the selection and names
it. A right click offers Create set from selection.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/ui-egui/src/capture.rs

@@ -84,6 +84,7 @@ impl ModelApp {
         let toolbar = std::mem::take(&mut self.toolbar_rects);
         let layout = std::mem::take(&mut self.layout);
         let panels = std::mem::take(&mut self.panel_rects);
+        self.capturing = true;
         crate::theme::apply(&off.ctx, self.ui.dark);
         // A capture is one still frame: windows and menus appear without fading in.
         off.ctx.global_style_mut(|s| s.animation_time = 0.0);
@@ -114,6 +115,7 @@ impl ModelApp {
         }
         std::mem::swap(&mut self.icons, &mut off.icons);
         std::mem::swap(&mut self.viewport, &mut off.viewport);
+        self.capturing = false;
         self.toolbar_rects = toolbar;
         self.layout = layout;
         self.panel_rects = panels;