UI: datum planes and axes in the viewport, datums and partitions in the browser

Datum planes draw as a hatched square around the model, axes as a line through
it, each with its name, in the desktop viewport and the headless render. The
browser's Geometry node lists the datums and the partitions with the datum
each cuts by.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/view.rs

@@ -72,6 +72,7 @@ impl Palette {
             interaction: Rgb(0x9a, 0x30, 0xc0),
             thermal: Rgb(0xe8, 0x4c, 0x00),
             label: Rgb(0x26, 0x2a, 0x32),
+            datum: Rgb(0x1e, 0x8c, 0x6e),
         },
     };
 
@@ -156,6 +157,7 @@ pub fn scene(s: &Session, p: &Palette) -> (Scene, Aabb3, Shown) {
         let mut bounds = base.bounds;
         let mut scene = model_scene(&base, p);
         bounds.union(glyphs.draw(|label| base.node_position(label), GlyphShow::ALL, &p.glyphs, &mut scene));
+        bounds.union(fieldcraft_render::glyphs::datums(&s.model, &base.bounds, &p.glyphs, &mut scene));
         return (scene, bounds, shown);
     };
     shown.step = Some(view.step.clone());