render: symbols for boundary conditions, loads and interactions, and scene labels

glyphs::Glyphs picks where a model's setup symbols go (resolving its sets
once, thinning them on a grid, capping each definition and sampling large
ones with a note) and draws them into a Scene at the nodes' current
positions: cones for constrained translations (and rotations where nodes
rotate), arrows for forces, double-headed arrows for moments, arrows normal
to pressure faces, one arrow for gravity and body forces, a curved arrow for
centrifugal loads, crosses with names for reference points, lines for
couplings, hatched squares for rigid walls. Colours come from the caller by
role, muted over results.

Scene gains labels, drawn by the CPU rasterizer with a small bitmap font of
its own.

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

crates/render/src/display.rs

@@ -103,7 +103,7 @@ fn face_key(e: &[u32], face: &[usize]) -> Option<[u32; 4]> {
 }
 
 /// Newell's normal of a polygon (robust for non-planar quads); `None` when degenerate.
-fn polygon_normal(p: &[Vec3]) -> Option<Vec3> {
+pub(crate) fn polygon_normal(p: &[Vec3]) -> Option<Vec3> {
     let mut n = Vec3::ZERO;
     for (i, a) in p.iter().enumerate() {
         let b = p.get((i + 1) % p.len()).copied().unwrap_or(*a);
@@ -112,7 +112,7 @@ fn polygon_normal(p: &[Vec3]) -> Option<Vec3> {
     n.normalized()
 }
 
-fn centre(p: &[Vec3]) -> Vec3 {
+pub(crate) fn centre(p: &[Vec3]) -> Vec3 {
     p.iter().fold(Vec3::ZERO, |s, q| s + *q) * (1.0 / p.len().max(1) as f64)
 }
 
@@ -243,6 +243,11 @@ impl DisplayMesh {
         }
     }
 
+    /// Where node `label` is drawn, if it is (nodes are kept in label order).
+    pub fn node_position(&self, label: u32) -> Option<Vec3> {
+        self.nodes.binary_search_by_key(&label, |n| n.label).ok().and_then(|i| self.nodes.get(i)).map(|n| n.p)
+    }
+
     /// The node nearest to where a ray (`origin`, `dir`) first hits an element face ahead of
     /// its origin: the hit face's closest corner, and the hit point. Line elements and geometry
     /// are not hit.