feat: add start-offset validation against file durations

extract.py

@@ -439,6 +439,24 @@ _stems = _derive_stems(INPUT_FILES)
 # ─── Prepare output folder ───
 os.makedirs(OUT_DIR, exist_ok=True)
 
+# ─── Validate start offset against file durations ───
+for _check_idx, _check_file in enumerate(INPUT_FILES):
+    try:
+        _r = subprocess.run(
+            ["ffprobe", "-v", "error", "-show_entries", "format=duration",
+             "-of", "default=noprint_wrappers=1:nokey=1", _check_file],
+            capture_output=True, text=True, timeout=10
+        )
+        _dur = float(_r.stdout.strip()) if _r.returncode == 0 else 0
+        _label = "A" if _check_idx == 0 else "B"
+        if _dur > 0 and START_SEC >= _dur:
+            print(f"WARNING: start={START_SEC}s is BEYOND file {_label} duration ({_dur:.1f}s) — extraction will produce empty/silent output!", file=sys.stderr)
+        elif _dur > 0 and START_SEC + DURATION > _dur:
+            _remaining = _dur - START_SEC
+            print(f"WARNING: start={START_SEC}s + duration={DURATION}s exceeds file {_label} ({_dur:.1f}s) — only {_remaining:.1f}s available", file=sys.stderr)
+    except Exception:
+        pass
+
 _audio_stream_selections = []  # track selected stream indices for config
 
 for _file_idx, INPUT_FILE in enumerate(INPUT_FILES):