perf: cache prereq versions and resolved stream URLs

The bundled yt-dlp is a PyInstaller one-file binary that unpacks ~37MB
on every invocation, costing about eight seconds a call on this machine
regardless of signing, xattrs or thinning the universal binary. Homebrew's
copy is a Python script and starts instantly, so bundling traded startup
speed for self-containment.

Softens it where possible: tool versions are read once at startup in the
background rather than on every Settings open, and resolved stream URLs
are cached for three hours (well inside YouTube's signed-URL lifetime) so
replaying a video costs nothing. A first play still pays the startup.

src-tauri/src/commands.rs

@@ -37,8 +37,17 @@ pub struct AppState {
     pub children: Arc<Mutex<HashMap<String, tokio::process::Child>>>,
     pub download_slots: Arc<Semaphore>,
     pub playlists: PlaylistServer,
+    /// yt-dlp's PyInstaller bundle costs several seconds to start, so its
+    /// version is read once and kept.
+    pub prereqs: Arc<Mutex<Option<Prereqs>>>,
+    /// Resolved stream URLs, keyed by video and quality cap. YouTube's signed
+    /// URLs last hours, so replaying a video should not pay for yt-dlp again.
+    pub streams: Arc<Mutex<HashMap<(String, Option<u32>), (String, std::time::Instant)>>>,
 }
 
+/// Comfortably inside the ~6h lifetime of YouTube's signed URLs.
+const STREAM_CACHE_TTL: std::time::Duration = std::time::Duration::from_secs(3 * 3600);
+
 #[derive(Clone, Serialize)]
 struct RefreshProgress {
     done: usize,
@@ -140,8 +149,16 @@ fn label(version: &str, name: &str) -> String {
 
 #[tauri::command]
 pub async fn check_prereqs(state: State<'_, AppState>) -> Result<Prereqs, String> {
+    // The library path can change, so only the tool versions are cached.
     let library = state.library.lock().await.clone();
-    Ok(Prereqs {
+    if let Some(cached) = state.prereqs.lock().await.clone() {
+        return Ok(Prereqs {
+            library_path: library.to_string_lossy().to_string(),
+            ..cached
+        });
+    }
+
+    let fresh = Prereqs {
         yt_dlp: version_of("yt-dlp", "--version").await.map(|v| label(&v, "yt-dlp")),
         ffmpeg: version_of("ffmpeg", "-version").await.map(|v| {
             // ffmpeg's first line is long; keep the useful head of it.
@@ -149,7 +166,9 @@ pub async fn check_prereqs(state: State<'_, AppState>) -> Result<Prereqs, String
             label(&head, "ffmpeg")
         }),
         library_path: library.to_string_lossy().to_string(),
-    })
+    };
+    *state.prereqs.lock().await = Some(fresh.clone());
+    Ok(fresh)
 }
 
 async fn read_channels(path: &str) -> Result<Vec<Channel>, String> {
@@ -227,6 +246,15 @@ pub async fn resolve_stream(
     max_height: Option<u32>,
     state: State<'_, AppState>,
 ) -> Result<Stream, String> {
+    let key = (video_id.clone(), max_height);
+    {
+        let mut cache = state.streams.lock().await;
+        cache.retain(|_, (_, at)| at.elapsed() < STREAM_CACHE_TTL);
+        if let Some((cached, _)) = cache.get(&key) {
+            return Ok(Stream { url: Some(cached.clone()), playlist: None });
+        }
+    }
+
     let url = format!("https://www.youtube.com/watch?v={video_id}");
 
     // The HLS master playlist. Every m3u8 format shares the same manifest_url,
@@ -237,6 +265,7 @@ pub async fn resolve_stream(
     .await
     {
         let Some(cap) = max_height else {
+            remember(&state, key, &master).await;
             return Ok(Stream { url: Some(master), playlist: None });
         };
         // Fetch and cut it down. If anything about that fails, the adaptive
@@ -244,10 +273,11 @@ pub async fn resolve_stream(
         return match state.http.get(&master).send().await {
             Ok(resp) => match resp.text().await {
                 Ok(body) => Ok(match filter_master_playlist(&body, cap) {
-                    Some(filtered) => Stream {
-                        url: Some(state.playlists.publish(filtered).await),
-                        playlist: None,
-                    },
+                    Some(filtered) => {
+                        let served = state.playlists.publish(filtered).await;
+                        remember(&state, key, &served).await;
+                        Stream { url: Some(served), playlist: None }
+                    }
                     None => Stream { url: Some(master), playlist: None },
                 }),
                 Err(_) => Ok(Stream { url: Some(master), playlist: None }),
@@ -272,6 +302,18 @@ pub async fn resolve_stream(
     Err("Could not find a playable stream for this video.".into())
 }
 
+async fn remember(
+    state: &State<'_, AppState>,
+    key: (String, Option<u32>),
+    url: &str,
+) {
+    state
+        .streams
+        .lock()
+        .await
+        .insert(key, (url.to_string(), std::time::Instant::now()));
+}
+
 /// Keeps only the highest video variant at or below `max_height`, along with
 /// every `EXT-X-MEDIA` line (the audio and subtitle groups it references).
 ///
@@ -766,6 +808,8 @@ pub fn build_state(app: &AppHandle) -> Result<AppState, String> {
         children: Arc::new(Mutex::new(HashMap::new())),
         download_slots: Arc::new(Semaphore::new(DOWNLOAD_CONCURRENCY)),
         playlists: PlaylistServer::start()?,
+        prereqs: Arc::new(Mutex::new(None)),
+        streams: Arc::new(Mutex::new(HashMap::new())),
     })
 }