fix: add flycast standalone data directory paths

The standalone build reads BIOS files from its data directory root, not from dc/ (core/oslib/oslib.cpp:118-196, core/stdclass.cpp:95-125, core/linux-dist/main.cpp:74-99). Each entry now carries standalone_path.

tests/test_flycast_layout.py

@@ -0,0 +1,31 @@
+"""Flycast's two builds read the same files from two directories.
+
+The libretro port looks in <system>/dc/ (shell/libretro/libretro.cpp:2249-2250).
+The standalone emulator looks in its data directory, whose root holds the
+files with no dc/ level (core/oslib/oslib.cpp:118-196, core/stdclass.cpp:95-125).
+Without standalone_path the standalone layout of a pack fell back on path:
+and put every file one directory too deep.
+"""
+
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+
+import yaml
+
+PROFILE = Path(__file__).resolve().parent.parent / "emulators" / "flycast.yml"
+
+
+class FlycastStandaloneLayout(unittest.TestCase):
+    def test_every_entry_has_the_data_directory_layout(self) -> None:
+        profile = yaml.safe_load(PROFILE.read_text(encoding="utf-8"))
+        self.assertEqual(profile["type"], "standalone + libretro")
+        for entry in profile["files"]:
+            with self.subTest(entry=entry["name"]):
+                self.assertTrue(entry["path"].startswith("dc/"))
+                self.assertEqual(entry.get("standalone_path"), entry["path"][len("dc/"):])
+
+
+if __name__ == "__main__":
+    unittest.main()