fix: data trees go with the systems they serve

Choosing GBA alone brought FBNeo's cheats and samples, 3,472 files,
because both trees listed every FBNeo system. The profile now names the
systems each serves, from the code: a driver's <name>.ini, cheat.dat,
cheatnes.dat or cheatsnes.dat (conc.cpp:815-828, 1059-1083), the md_,
pce_, tg_ and sgx_ drivers the tree holds, and samples read by arcade
drivers alone (samples.cpp:886-899).

The list now gives a tree the platform's ids its systems normalize to,
as a core extra's: the cheats said sega-megadrive where RetroArch says
sega-mega-drive, and a tree no platform system serves goes with the
files of none, as the pack puts it. The installer keeps a tree for the
systems chosen by name, not for every system a shared file names: the
arcade choice took blueMSX's machines through msx.zip.

tests/test_install_selection.py

@@ -913,6 +913,57 @@ class InstallMatchesASystemPack(unittest.TestCase):
                 )
 
 
+class DataTreesFollowTheSystemsTheyServe(InstallMatchesASystemPack):
+    """The data trees a system choice keeps, installer against pack. GBA
+    alone took FBNeo's cheats and samples, 3,472 files, because the trees
+    listed every FBNeo system. The Mega Drive took no cheat: the tree said
+    sega-megadrive where RetroArch's list says sega-mega-drive."""
+
+    CASES = ((["nintendo-gba"], []), (["sega-mega-drive"], []), (["arcade"], []))
+
+    def _packed(self, systems, regions) -> set[str]:
+        import zipfile
+
+        from common import load_data_dir_registry
+
+        # The trees come from the registry the manifest reads by itself.
+        with tempfile.TemporaryDirectory(dir=REPO_ROOT / "tmp") as out:
+            zip_path = self.gp.generate_pack(
+                self.PLATFORM, str(REPO_ROOT / "platforms"), self.db,
+                str(REPO_ROOT / "bios"), out,
+                emulators_dir=str(REPO_ROOT / "emulators"),
+                emu_profiles=self.profiles, system_filter=systems,
+                regions=regions or None, offline=True,
+                data_registry=load_data_dir_registry(str(REPO_ROOT / "platforms")),
+            )
+            with zipfile.ZipFile(zip_path) as zf:
+                names = set(zf.namelist())
+        prefix = f"{self.base}/" if self.base else ""
+        return {n[len(prefix):] if prefix and n.startswith(prefix) else n for n in names}
+
+    def _members(self, trees) -> set[str]:
+        return {f"{tree['dest']}/{rel}" if tree["dest"] else rel
+                for tree in trees for rel in tree["files"]}
+
+    def test_each_case(self):
+        everything = self._members(self.full.get("data_directories") or [])
+        for systems, regions in self.CASES:
+            with self.subTest(systems=systems):
+                files, _omitted = install.narrow(
+                    self.full["files"], self.full["omitted_files"], systems, [], regions)
+                trees = install.select_data_directories(self.full, files, True, systems=systems)
+                refs = {tree["ref"] for tree in trees}
+                if systems == ["nintendo-gba"]:
+                    self.assertFalse({"fbneo-cheats", "fbneo-samples"} & refs)
+                if systems == ["sega-mega-drive"]:
+                    self.assertIn("fbneo-cheats", refs)
+                    self.assertNotIn("fbneo-samples", refs)
+                if systems == ["arcade"]:
+                    self.assertTrue({"fbneo-cheats", "fbneo-samples"} <= refs)
+                self.assertEqual(self._members(trees),
+                                 self._packed(systems, regions) & everything)
+
+
 class InstallMatchesATargetManifest(unittest.TestCase):
     """`install.py --target T` against the manifest the builder writes for T.