Setup symbols: wireframe glyphs per held degree of freedom, a constant size on screen

Boundary condition, load and interaction symbols are wireframes (thin lines,
no shaded cones), so they never hide the mesh or a contour plot:

- a crossed wire triangle per constrained translation, tip on the node,
  coming from outside the part where the set's faces say which side that is;
  a held rotation adds a smaller triangle behind it (the double mark); fixed
  shows all three axes, a symmetry plane only its normal;
- forces as open-headed arrows ending on the node, as long as four support
  units at one place and 2.5 over a set, so loads and supports stay in
  proportion (#78); moments as an axis line and a right-handed curved arrow
  (#79); contact and tie surfaces as small squares on their faces;
- a constant size on screen at any zoom (#80): the symbol unit is the size
  setting (View > Symbol size, `ui.set {symbol_size}`, 14 pt by default)
  times the model length a pixel spans, rounded to a quarter of a doubling;
- spread evenly on a grid a few symbol units across, so their density follows
  the screen, never more than 400 per definition;
- a layer of their own on the desktop (GPU and CPU): zooming rebuilds and
  uploads the symbols only, never the mesh's scene; framing leaves them out;
- colour per kind from the theme's tokens (a new contact colour), and the
  definition selected in the browser drawn in the selection colour, wider;
- depth-tested against the model with the line bias that keeps them on their
  own surface; a load at a reference point or a single node and a reference
  point's cross are drawn over the part (#78).

Headless pictures (MCP screenshot, `ui.render`) frame the camera first and
draw the symbols at the default size for it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/render/src/raster.rs

@@ -67,6 +67,17 @@ pub struct Scene {
     pub radius: f64,
 }
 
+impl Scene {
+    /// Add `other`'s primitives (not its background or radius) to this scene.
+    pub fn append(&mut self, other: Scene) {
+        self.tris.extend(other.tris);
+        self.lines.extend(other.lines);
+        self.points.extend(other.points);
+        self.labels.extend(other.labels);
+        self.on_top.extend(other.on_top);
+    }
+}
+
 /// The lit colour of a surface with normal `n` seen along `view_dir` (toward the eye). The GPU
 /// shader computes the same.
 pub fn shade(n: Vec3, view_dir: Vec3, base: [f64; 3]) -> [f64; 3] {