fix: games tools write whole files, name options

The games importers wrote through fixed .part names, the new options had
no help text, and romfolders stopped on the first setup the offline cache
lacked. RetroArch's rom folder map is regenerated from its source spec
(path null, as the schema says). The installer's Python 3.8 job imports
the games tests without pyyaml, so the region rules load lazily.

scripts/artifacts.py

@@ -7,6 +7,7 @@ it is only meaningful if the comparison ignores the hour."""
 from __future__ import annotations
 
 from collections.abc import Callable, Iterator
+from typing import BinaryIO
 import contextlib
 import json
 import os
@@ -159,6 +160,29 @@ def copy_file_atomic(source: str, path: str) -> None:
             os.unlink(scratch)
         raise
 
+
+def copy_stream_atomic(stream: BinaryIO, path: str) -> None:
+    """Stream a download into place whole or not at all.
+
+    A fixed scratch name let two runs interleave their bytes in one file of
+    the right size, which the rename then promoted."""
+    import shutil
+
+    directory = os.path.dirname(os.path.abspath(path))
+    handle, scratch = tempfile.mkstemp(
+        dir=directory, prefix=f".{os.path.basename(path)}.", suffix=".tmp"
+    )
+    try:
+        with os.fdopen(handle, "wb") as f:
+            shutil.copyfileobj(stream, f)
+        set_plain_mode(scratch)
+        os.replace(scratch, path)
+    except BaseException:
+        with contextlib.suppress(OSError):
+            os.unlink(scratch)
+        raise
+
+
 def _strip_timestamps(text: str) -> str:
     """Remove known timestamp patterns for content comparison."""
     result = text