fix: create tmp/tests when the test package loads

Most modules write under tmp/, which a fresh clone lacks, and only three
created it on import. A module run alone failed with FileNotFoundError on
a temporary directory.

tests/__init__.py

@@ -4,10 +4,19 @@ The CI job sets RETROBIOS_OFFLINE, but a local run of the documented command
 did not, and e2e fixtures queried the large-files release and wrote its
 answers into the shared .cache/large. A test that serves its own assets
 clears the variable for itself.
+
+Scratch files go under the repository's tmp/, which git does not track: a
+fresh clone has none, and a module run alone raised FileNotFoundError on a
+temporary directory instead of naming the missing parent.
 """
 
 from __future__ import annotations
 
 import os
+from pathlib import Path
 
 os.environ.setdefault("RETROBIOS_OFFLINE", "1")
+
+(Path(__file__).resolve().parent.parent / "tmp" / "tests").mkdir(
+    parents=True, exist_ok=True
+)

tests/test_audit_regressions.py

@@ -166,6 +166,27 @@ class ReadmeRegressions(unittest.TestCase):
         self.assertEqual(failed, {})
 
 
+class AModuleRunAloneHasItsScratchDirectory(unittest.TestCase):
+    """About forty modules write under tmp/, which git does not track. Only
+    three created it, as an import side effect, so a module run alone on a
+    fresh clone raised FileNotFoundError. The test package creates it."""
+
+    def test_importing_the_package_creates_tmp(self):
+        with tempfile.TemporaryDirectory(dir=TMP_ROOT) as directory:
+            clone = Path(directory)
+            (clone / "tests").mkdir()
+            (clone / "tests" / "__init__.py").write_bytes(
+                (ROOT / "tests" / "__init__.py").read_bytes()
+            )
+            proc = subprocess.run(
+                [sys.executable, "-I", "-c",
+                 "import sys; sys.path.insert(0, '.'); import tests"],
+                cwd=clone, capture_output=True, text=True, timeout=60, check=False,
+            )
+            self.assertEqual(proc.returncode, 0, proc.stderr)
+            self.assertTrue((clone / "tmp" / "tests").is_dir())
+
+
 class ReleaseSigningRegressions(unittest.TestCase):
     """A checksum list published beside its own artifacts proves nothing.