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/ui-egui/src/theme.rs

@@ -82,6 +82,8 @@ pub struct Tokens {
     pub glyph_interaction: Color32,
     /// Temperatures, heat fluxes, convection and heat sources.
     pub glyph_thermal: Color32,
+    /// Datum planes and axes.
+    pub glyph_datum: Color32,
     // ---- selection ----
     /// What is selected in the viewport, and what a click would pick under the pointer.
     pub pick_selected: Color32,
@@ -159,6 +161,7 @@ impl Tokens {
             glyph_load: Color32::from_rgb(0xd0, 0x24, 0x24),
             glyph_interaction: Color32::from_rgb(0x9a, 0x30, 0xc0),
             glyph_thermal: Color32::from_rgb(0xe8, 0x4c, 0x00),
+            glyph_datum: Color32::from_rgb(0x1e, 0x8c, 0x6e),
             pick_selected: Color32::from_rgb(0xd6, 0x00, 0x7f),
             pick_hover: Color32::from_rgb(0xf0, 0x7c, 0xc0),
         }
@@ -221,6 +224,7 @@ impl Tokens {
         t.glyph_load = Color32::from_rgb(0xff, 0x5a, 0x4c);
         t.glyph_interaction = Color32::from_rgb(0xd2, 0x82, 0xff);
         t.glyph_thermal = Color32::from_rgb(0xff, 0x8c, 0x00);
+        t.glyph_datum = Color32::from_rgb(0x5a, 0xd8, 0xaa);
         t.pick_selected = Color32::from_rgb(0xff, 0x3f, 0xb4);
         t.pick_hover = Color32::from_rgb(0xff, 0xa6, 0xdc);
         t
@@ -239,6 +243,7 @@ impl Tokens {
             interaction: rgb(self.glyph_interaction),
             thermal: rgb(self.glyph_thermal),
             label: rgb(self.overlay_text),
+            datum: rgb(self.glyph_datum),
         }
     }