Implement the reviewed UI icon system and durable review workflow (#853)

Thanks @Nathan-Dane.

crates/ui-egui/src/theme.rs

@@ -3,7 +3,6 @@
 //! uses comes from [`Tokens`].
 
 use std::cell::Cell;
-use std::sync::atomic::{AtomicBool, Ordering};
 
 #[path = "../../../assets/styles/ui/palettes.rs"]
 mod palettes;
@@ -11,10 +10,11 @@ mod palettes;
 use egui::{Color32, Stroke};
 use serde::{Deserialize, Serialize};
 
-static DARK: AtomicBool = AtomicBool::new(true);
-
 thread_local! {
-    /// The colour vision mode, per thread, where the dark theme is one global (`DARK`). Each app
+    /// Theme state belongs to the UI thread drawing the frame. Concurrent app/test renderers
+    /// must not change one another's palette while a frame or icon texture is being built.
+    static DARK: Cell<bool> = const { Cell::new(true) };
+    /// The colour vision mode, per thread. Each app
     /// sets it from its own preferences at the start of every frame (`SolveApp::logic`), on the
     /// thread that draws it. Tests run many apps side by side on their own threads in one process:
     /// with one global, a test in a mode changed the colours under another one
@@ -26,7 +26,12 @@ thread_local! {
 
 /// Is the dark theme on?
 pub fn is_dark() -> bool {
-    DARK.load(Ordering::Relaxed)
+    DARK.get()
+}
+
+/// The common conversion boundary for compiled icon palette data.
+pub(crate) const fn icon_color(rgb: [u8; 3]) -> Color32 {
+    Color32::from_rgb(rgb[0], rgb[1], rgb[2])
 }
 
 /// Colour vision modes (#778): a second set of values for the colours that carry a meaning, for
@@ -333,7 +338,7 @@ pub fn material_color(name: &str) -> Option<Color32> {
 
 /// Switch themes (the visuals follow on the next [`apply`]).
 pub fn set_dark(dark: bool) {
-    DARK.store(dark, Ordering::Relaxed);
+    DARK.set(dark);
 }
 
 fn visuals(dark: bool) -> egui::Visuals {
@@ -373,6 +378,24 @@ pub fn apply(ctx: &egui::Context) {
 
 #[cfg(test)]
 mod tests {
+    #[test]
+    fn concurrent_ui_renderers_keep_their_own_theme_and_vision() {
+        let barrier = std::sync::Barrier::new(2);
+        std::thread::scope(|scope| {
+            for (dark, vision) in [(false, super::ColorVision::Off), (true, super::ColorVision::Protan)] {
+                let barrier = &barrier;
+                scope.spawn(move || {
+                    super::set_dark(dark);
+                    super::set_vision(vision);
+                    barrier.wait();
+                    assert_eq!(super::is_dark(), dark);
+                    assert_eq!(super::vision(), vision);
+                    assert_eq!(super::Tokens::get().sketch_accent, super::Tokens::themed(dark, vision).sketch_accent);
+                });
+            }
+        });
+    }
+
     /// Title and section bands stand out from a dialog, the title more than its sections.
     #[test]
     fn heading_bands_are_darker_than_the_dialog_in_order() {