Clear clippy 1.97 lints across bread-theme, bread-utils, bread-polkit

Toolchain drift (clippy 0.1.97): pre-existing on main, all mechanical
and behaviour-preserving.

- bread-theme/gtk.rs: `type AppCssBuilder` alias for the repeated
  `Rc<dyn Fn(&Palette) -> String>` (type_complexity ×4).
- bread-theme/lib.rs: `sort_by_key(|..| Reverse(len))` (unnecessary_sort_by).
- bread-theme/output.rs: `io::Error::other`; struct-init in a test.
- bread-utils/singleton.rs: explicit `.truncate(false)` on the lock file
  open — we only clear it after winning the lock (suspicious_open_options).
- bread-polkit/identity.rs: elide `pick_user` lifetimes.

bread-theme/src/gtk.rs

@@ -10,6 +10,10 @@ use std::rc::Rc;
 
 use crate::Palette;
 
+/// Per-widget app-CSS builder: given the resolved palette for the widget's
+/// monitor, produce the app stylesheet to layer on top of the theme CSS.
+type AppCssBuilder = Rc<dyn Fn(&Palette) -> String>;
+
 /// Above APPLICATION (600) so we beat [`apply_shared`], below USER (800)
 /// so `apply_user_css` still wins.
 const BIND_PRIORITY: u32 = gtk4::STYLE_PROVIDER_PRIORITY_USER - 10;
@@ -162,7 +166,7 @@ struct WidgetBind {
     output: String,
     theme: CssProvider,
     app: Option<CssProvider>,
-    app_build: Option<Rc<dyn Fn(&Palette) -> String>>,
+    app_build: Option<AppCssBuilder>,
     /// Keep the directory monitor + child model alive for this widget.
     _watch: Option<gio::ListModel>,
 }
@@ -284,7 +288,7 @@ fn watch_root_children(widget: &gtk4::Widget) -> gio::ListModel {
 fn bind_window_inner(
     widget: &gtk4::Widget,
     output: &str,
-    app_build: Option<Rc<dyn Fn(&Palette) -> String>>,
+    app_build: Option<AppCssBuilder>,
 ) {
     let key = widget_key(widget);
     let palette = crate::load_palette_for(output);
@@ -387,7 +391,7 @@ where
     bind_window_inner(widget.as_ref(), output, Some(Rc::new(build)));
 }
 
-fn attach_enter_monitor(widget: &gtk4::Widget, build: Option<Rc<dyn Fn(&Palette) -> String>>) {
+fn attach_enter_monitor(widget: &gtk4::Widget, build: Option<AppCssBuilder>) {
     let Some(native) = widget.native() else {
         return;
     };
@@ -408,7 +412,7 @@ fn attach_enter_monitor(widget: &gtk4::Widget, build: Option<Rc<dyn Fn(&Palette)
     });
 }
 
-fn bind_auto(native: &gtk4::Native, build: Option<Rc<dyn Fn(&Palette) -> String>>) {
+fn bind_auto(native: &gtk4::Native, build: Option<AppCssBuilder>) {
     let widget = native.upcast_ref::<gtk4::Widget>().clone();
 
     let apply = {

bread-theme/src/lib.rs

@@ -330,7 +330,8 @@ pub fn stylesheet_resolved(p: &Palette) -> String {
 /// named colors cannot leak the wrong monitor's accent.
 pub(crate) fn resolve_color_names(css: &str, p: &Palette) -> String {
     let mut pairs: Vec<(&str, String)> = color_pairs(p).into_iter().collect();
-    pairs.sort_by(|a, b| b.0.len().cmp(&a.0.len()));
+    // Longest name first, so `@on-bg` is replaced before `@bg` can match its tail.
+    pairs.sort_by_key(|(name, _)| std::cmp::Reverse(name.len()));
     let mut out = css.to_string();
     for (name, value) in pairs {
         out = out.replace(&format!("@{name}"), &value);
@@ -544,8 +545,10 @@ mod tests {
 
     #[test]
     fn stylesheet_resolved_inlines_color4_and_drops_named_refs_in_rules() {
-        let mut p = Palette::default();
-        p.color4 = "#7aa2f7".into();
+        let p = Palette {
+            color4: "#7aa2f7".into(),
+            ..Default::default()
+        };
         let css = stylesheet_resolved(&p);
         assert!(css.contains("#7aa2f7"), "color4 must appear as hex: {css}");
         // Rule bodies must not keep named colors — GTK display-global

bread-theme/src/output.rs

@@ -185,10 +185,7 @@ pub fn palette_from_image(path: &Path) -> std::io::Result<Palette> {
         Ok(s) => s,
     };
     if !status.success() {
-        return Err(std::io::Error::new(
-            std::io::ErrorKind::Other,
-            format!("wal failed with {status}"),
-        ));
+        return Err(std::io::Error::other(format!("wal failed with {status}")));
     }
 
     let json_path = [
@@ -299,9 +296,11 @@ mod tests {
     #[test]
     fn write_output_palette_roundtrips_color4() {
         with_runtime_dir(|_| {
-            let mut p = Palette::default();
-            p.color4 = "#7aa2f7".into();
-            p.background = "#ffffff".into();
+            let p = Palette {
+                color4: "#7aa2f7".into(),
+                background: "#ffffff".into(),
+                ..Default::default()
+            };
             write_output_palette("HDMI-A-1", &p).unwrap();
             let loaded = load_palette_for("HDMI-A-1");
             assert_eq!(loaded.color4, "#7aa2f7");