V42+V42A: fix seam click khi loop master FX — worklet wrap crossfade (tail->head) + native WebAudio loop cho bypass (het click beat 0); dip bypass 5ms; PDC playhead freeze/snap

src-tauri/src/lib.rs

@@ -293,7 +293,8 @@ pub fn run() {
             bridge_status,
             transport_control,
             set_fx_chain,
-            restart_bridge_with_sample_rate
+            restart_bridge_with_sample_rate,
+            dbg_log
         ])
         .setup(|app| {
             // V10 (FIX_DRAG_DROP_WINDOWS): disable Tauri's built-in drag-drop
@@ -900,6 +901,27 @@ fn bridge_status(app: AppHandle) -> Result<BridgeStatus, String> {
     Ok(status)
 }
 
+/// V28: JS timing diagnostics — append `{unix_ms} {tag} {extra}` to
+/// %APPDATA%/SonicForgeDAW/logs/dbg.log (create dirs as needed). Fire-and-forget
+/// từ app.jsx (_dbgLog) để định vị delay Play/Stop không cần devtools.
+#[tauri::command]
+fn dbg_log(tag: String, extra: String) -> Result<(), String> {
+    let Some(appdata) = std::env::var("APPDATA").ok() else { return Ok(()) };
+    let dir = Path::new(&appdata).join("SonicForgeDAW").join("logs");
+    let _ = std::fs::create_dir_all(&dir);
+    let path = dir.join("dbg.log");
+    let now_ms = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .map(|d| d.as_millis())
+        .unwrap_or(0);
+    let line = format!("{} {} {}\n", now_ms, tag, extra);
+    use std::io::Write;
+    if let Ok(mut f) = std::fs::OpenOptions::new().create(true).append(true).open(&path) {
+        let _ = f.write_all(line.as_bytes());
+    }
+    Ok(())
+}
+
 #[tauri::command]
 fn transport_control(app: AppHandle, kind: String, playhead: Option<u32>) -> Result<(), String> {
     let state = app.state::<ShmState>();