fix: give system packs their core extras

tests/test_system_pack_extras.py

@@ -0,0 +1,36 @@
+"""A pack limited to some systems carries those systems' core extras.
+
+With --platform and --system, the full source emptied the core extras, so
+`--source full` and `--source platform` built the same pack under two
+names; the split packs, built per system, carried them.
+"""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+from generate_pack import _extras_for_systems  # noqa: E402
+
+
+class SystemExtras(unittest.TestCase):
+    def test_extras_follow_their_system(self):
+        extras = [
+            {"name": "a", "source_system": "sony-playstation"},
+            {"name": "b", "source_system": "sega-saturn"},
+            {"name": "c", "source_systems": ["sony-playstation", "sony-psp"]},
+        ]
+        picked = [e["name"] for e in _extras_for_systems(extras, ["sony-playstation"])]
+        self.assertEqual(picked, ["a", "c"])
+
+    def test_full_source_does_not_empty_them(self):
+        source = (REPO_ROOT / "scripts" / "generate_pack.py").read_text(encoding="utf-8")
+        self.assertNotIn('elif system_filter and source != "truth":\n          core_files = []', source)
+
+
+if __name__ == "__main__":
+    unittest.main()