fix: apply_sync ffmpeg negative delay

apply_sync.py

@@ -84,10 +84,15 @@ def build_ffmpeg_cmd(file_a: str, file_b: str, lag_sec: float, out_path: str) ->
     set_german = (len(b_streams) == 1 and b_streams[0]["language"] in ("und", "unknown", ""))
 
     # Build filter_complex for B audio streams
-    # Negative adelay trims the start (B ahead of A), positive adds silence (B behind A)
+    # Positive lag (B behind A): adelay adds silence at start
+    # Negative lag (B ahead of A): atrim cuts the start, asetpts resets timestamps
     filters = []
     for i, s in enumerate(b_streams):
+        if lag_ms >= 0:
             filters.append(f"[1:a:{i}]adelay={lag_ms:.0f}|{lag_ms:.0f}:all=1[d{i}]")
+        else:
+            trim_sec = -lag_sec
+            filters.append(f"[1:a:{i}]atrim=start={trim_sec:.6f},asetpts=N/SR/TB[d{i}]")
 
     cmd = ["ffmpeg", "-y", "-i", file_a, "-i", file_b]