Cap workers at cores-1 so encoding never stalls the system

ppl2psd.py

@@ -739,8 +739,16 @@ def build_psd(layers, width: int, height: int, out_path: str, workers: int,
 # --------------------------------------------------------------------------
 # Main
 # --------------------------------------------------------------------------
+def _safe_workers(workers: int) -> int:
+    """Never use every CPU core: leave one for the system, otherwise the
+    RLE-encoding workers can stall the whole device."""
+    available = max(1, (os.cpu_count() or 2) - 1)
+    return max(1, min(workers, available))
+
+
 def convert(ppl_path: str, out_path: str, preview_path: Optional[str] = None,
             workers: int = 4, type_layer: bool = False) -> None:
+    workers = _safe_workers(workers)
     import tempfile
     with tempfile.TemporaryDirectory() as tmp:
         with zipfile.ZipFile(ppl_path) as zf:
@@ -884,8 +892,9 @@ def main():
     ap.add_argument("input", help="input .ppl file")
     ap.add_argument("-o", "--output", help="output .psd path")
     ap.add_argument("--preview", help="also write a downscaled composite PNG")
-    ap.add_argument("-j", "--workers", type=int, default=4,
-                    help="parallel workers for RLE encoding (default 4)")
+    ap.add_argument("-j", "--workers", type=int,
+                    default=max(1, (os.cpu_count() or 2) - 1),
+                    help="parallel workers for RLE encoding (default: cores-1)")
     ap.add_argument("--type-layer", action="store_true",
                     help="convert text layers to editable TypeLayers (TySh) "
                          "instead of rasterized pixels")