fix(review): bugs, arch violations, design smells

P1 bugs:
- unix_launcher: shell_split respects quoted args (was split_whitespace)
- plugin-host: 5s timeout on external plugin search
- ui: handle engine init panic, wire error state
- ui-egui: read window config instead of always using defaults
- plugin-url: use OpenPath action instead of SpawnProcess+xdg-open

Architecture:
- remove WindowConfig (mirror of WindowCfg); use WindowCfg directly
- remove on_select closure from SearchResult (domain leakage)
- remove LaunchAction::Custom; add Plugin::on_selected + SearchEngine::on_selected
- apps: record frecency via on_selected instead of embedded closure

Design smells:
- frecency: extract decay_factor helper, write outside mutex
- apps: remove cfg(test) cache_path hack; add new_for_test ctor
- apps: stable ResultId using name+exec to prevent collision
- files: stable ResultId using full path instead of index
- plugin-host: remove k-launcher-os-bridge dep (WindowConfig gone)

crates/k-launcher-kernel/src/lib.rs

@@ -50,7 +50,6 @@ pub enum LaunchAction {
     SpawnInTerminal(String),
     OpenPath(String),
     CopyToClipboard(String),
-    Custom(Arc<dyn Fn() + Send + Sync>),
 }
 
 // --- AppLauncher port trait ---
@@ -68,7 +67,6 @@ pub struct SearchResult {
     pub icon: Option<String>,
     pub score: Score,
     pub action: LaunchAction,
-    pub on_select: Option<Arc<dyn Fn() + Send + Sync>>,
 }
 
 impl std::fmt::Debug for SearchResult {
@@ -88,6 +86,7 @@ impl std::fmt::Debug for SearchResult {
 pub trait Plugin: Send + Sync {
     fn name(&self) -> &str;
     async fn search(&self, query: &str) -> Vec<SearchResult>;
+    fn on_selected(&self, _id: &ResultId) {}
 }
 
 // --- SearchEngine port trait ---
@@ -95,6 +94,7 @@ pub trait Plugin: Send + Sync {
 #[async_trait]
 pub trait SearchEngine: Send + Sync {
     async fn search(&self, query: &str) -> Vec<SearchResult>;
+    fn on_selected(&self, id: &ResultId);
 }
 
 // --- NullSearchEngine ---
@@ -106,6 +106,7 @@ impl SearchEngine for NullSearchEngine {
     async fn search(&self, _query: &str) -> Vec<SearchResult> {
         vec![]
     }
+    fn on_selected(&self, _id: &ResultId) {}
 }
 
 // --- Kernel (Application use case) ---
@@ -123,6 +124,12 @@ impl Kernel {
         }
     }
 
+    pub fn on_selected(&self, id: &ResultId) {
+        for plugin in &self.plugins {
+            plugin.on_selected(id);
+        }
+    }
+
     pub async fn search(&self, query: &str) -> Vec<SearchResult> {
         use futures::FutureExt;
         use std::panic::AssertUnwindSafe;
@@ -154,6 +161,9 @@ impl SearchEngine for Kernel {
     async fn search(&self, query: &str) -> Vec<SearchResult> {
         self.search(query).await
     }
+    fn on_selected(&self, id: &ResultId) {
+        self.on_selected(id);
+    }
 }
 
 // --- Tests ---
@@ -188,8 +198,7 @@ mod tests {
                     description: None,
                     icon: None,
                     score: Score::new(*score),
-                    action: LaunchAction::Custom(Arc::new(|| {})),
-                    on_select: None,
+                    action: LaunchAction::SpawnProcess("mock".to_string()),
                 })
                 .collect()
         }