fix: copy standalone files from their pack path

The registry names one path per standalone copy for every platform,
but each manifest places the file its own way, so the installer
looked where the pack had put nothing and copied nothing: EmuDeck's
aes_keys.txt for Azahar, Batocera's switch/prod.keys. Each manifest
copy now names its source, which the installer validates and reads.

tests/test_install.py

@@ -882,6 +882,44 @@ class TestStandaloneCopiesExtraDirs(unittest.TestCase):
         )
 
 
+class StandaloneCopiesReadWhereThePackPutTheFile(unittest.TestCase):
+    """The registry names Citra/sysdata/aes_keys.txt for every platform;
+    EmuDeck's manifest installs it at the root. The installer looked under
+    the registry's path, found nothing and copied nothing to Azahar."""
+
+    def _copy(self, entry: dict) -> tuple[Path, tuple[int, int, int]]:
+        bios = Path(_tempdir(self))
+        (bios / "aes_keys.txt").write_text("keys")
+        sysdata = Path(_tempdir(self)) / "sysdata"
+        sysdata.mkdir()
+        manifest = {"standalone_copies": [
+            {**entry, "emulator": "azahar", "targets": {"linux": [str(sysdata)]}},
+        ]}
+        return sysdata, install.do_standalone_copies(manifest, bios, "linux")
+
+    def test_the_source_the_manifest_names_is_copied(self):
+        sysdata, (copied, _, _) = self._copy(
+            {"file": "Citra/sysdata/aes_keys.txt", "source": "aes_keys.txt"}
+        )
+        self.assertEqual(copied, 1)
+        self.assertEqual((sysdata / "aes_keys.txt").read_text(), "keys")
+
+    def test_without_a_source_the_file_path_is_read(self):
+        _, (copied, _, _) = self._copy({"file": "Citra/sysdata/aes_keys.txt"})
+        self.assertEqual(copied, 0)
+
+    def test_an_unsafe_source_is_refused_at_load(self):
+        manifest = json.loads(
+            (Path(__file__).resolve().parents[1] / "install" / "emudeck.json").read_text()
+        )
+        manifest["standalone_copies"] = [
+            {"file": "aes_keys.txt", "source": "../../etc/passwd",
+             "targets": {"linux": ["/tmp"]}},
+        ]
+        with self.assertRaisesRegex(ValueError, r"standalone_copies\[0\]\.source"):
+            install._validate_manifest(manifest, "emudeck")
+
+
 class InstallerStaysOnPython38AndTheStdlib(unittest.TestCase):
     """install.py runs where the bootstraps let it run: Python 3.8, stdlib only.