Image Trace: look up library swatches once per trace

Each traced path filtered the whole library for its color, twice when
no swatch of that color was linked. The color-to-paint lookup is now
built once before the paths are filled. The first linked swatch of a
color still gives the fill, and the first swatch of that color
otherwise.

crates/engine/src/cmd/buildcmds/trace.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use vectorcraft_color::Swatch;
 use vectorcraft_doc::ImageObject;
 
@@ -557,6 +559,16 @@ pub(super) fn trace_make(s: &mut Session, p: &Value, expand: bool) -> Result<Val
     let unit = xf.determinant().abs().sqrt();
     let (npaths, anchors, colors) = (res.paths.len(), res.anchor_count(), res.palette.len());
     let label = if expand { "Image Trace (Make and Expand)" } else { "Image Trace" };
+    // A library color fills with the swatch's own color (its model kept), linked to a
+    // global or spot swatch of the document. Of the swatches that trace as this color,
+    // the first linked one is taken, else the first. Build the lookup once per trace.
+    let mut by_color: HashMap<[u8; 3], &Paint> = HashMap::new();
+    for (rgb, paint) in &library {
+        let chosen = by_color.entry(*rgb).or_insert(paint);
+        if !matches!(**chosen, Paint::Solid { swatch: Some(_), .. }) && matches!(paint, Paint::Solid { swatch: Some(_), .. }) {
+            *chosen = paint;
+        }
+    }
     let id = s.edit(label, |d, sel| {
         let src = d.node(target).cloned().ok_or(EngineError::NoNode(target))?;
         let mut image = if is_trace_group(&src) {
@@ -584,14 +596,8 @@ pub(super) fn trace_make(s: &mut Session, p: &Value, expand: bool) -> Result<Val
                 *path = shape.to_path();
                 *live = Some(Box::new(shape));
             }
-            // A library color fills with the swatch's own color (its model kept), linked to a
-            // global or spot swatch of the document. Of the swatches that trace as this color,
-            // the first linked one is taken, else the first.
-            let same = || library.iter().filter(|(rgb, _)| *rgb == tp.color);
-            let solid = same()
-                .find(|(_, p)| matches!(p, Paint::Solid { swatch: Some(_), .. }))
-                .or_else(|| same().next())
-                .map_or_else(|| Paint::solid(Color::rgb8(tp.color[0], tp.color[1], tp.color[2])), |l| l.1.clone());
+            let solid =
+                by_color.get(&tp.color).map_or_else(|| Paint::solid(Color::rgb8(tp.color[0], tp.color[1], tp.color[2])), |paint| (*paint).clone());
             // Gradients: a path the tracer found to be a linear gradient is filled with it.
             let paint = tp.gradient.as_ref().map_or(solid, |g| traced_gradient(g, xf));
             n.appearance = match tp.stroke {