Add bread-launcher: headless app-launcher core + GTK4 results widget

Extracts the app-launcher substance (desktop-entry parsing, fuzzy
matching/ranking, launch history, launching) out of breadbox into a
reusable ecosystem crate, so breadbar's future embedded capsule and
breadbox's overlay window can share one implementation instead of two
(THEME_SYSTEM_PLAN.md Phase 4/6a). The GTK4 results-list widget lives
behind a `gtk` feature, mirroring bread-theme's `gtk`/`adw` gating, so a
headless consumer isn't forced to link GTK.

Adds unit tests for fuzzy_score/matches_term/priority_rank, which were
previously untested pure functions inside breadbox's main.rs.

bread-launcher/src/icon.rs

@@ -0,0 +1,26 @@
+use std::{fs, path::PathBuf};
+
+pub struct IconCache {
+    pub dir: PathBuf,
+}
+
+impl IconCache {
+    /// `app` picks the cache subdirectory (see [`crate::cache_dir`]) — pass
+    /// the same name across a process's calls so `path_for` and
+    /// `manifest_path` agree on where icons live.
+    pub fn new(app: &str) -> Self {
+        IconCache { dir: crate::paths::cache_dir(app).join("icons") }
+    }
+
+    pub fn path_for(&self, icon_name: &str) -> PathBuf {
+        self.dir.join(format!("{}.png", icon_name))
+    }
+
+    pub fn manifest_path(app: &str) -> PathBuf {
+        crate::paths::cache_dir(app).join("manifest.json")
+    }
+
+    pub fn ensure_dir(&self) -> std::io::Result<()> {
+        fs::create_dir_all(&self.dir)
+    }
+}