fix: judge a declared slot by the platform's copy

--declared-only settled each slot on its first declaration before the
cores narrowed the list, and the declared event judged the core copy
served in its place. Both now read the declaration whose systems
remain, as the platform checks it.

tests/test_install_cores.py

@@ -288,6 +288,56 @@ class APlatformFileSettledByCoresAndSystems(unittest.TestCase):
         self.assertEqual(self.served(["dsi"]), "c")
         self.assertEqual(self.served(["nds"]), "b")
 
+    def test_declared_only_keeps_the_declaration_whose_system_remains(self):
+        """--declared-only settled the slot on the first declaration before
+        the cores narrowed it: on a target without melonds it checked the
+        nintendo-dsi copy, the platform the nintendo-ds one."""
+        manifest = self.manifest()
+        for names, letter in ((["nds"], "b"), (["dsi"], "c"), (["melonds"], "c"),
+                              (["desmume"], "b")):
+            with self.subTest(names=names):
+                files, omitted = install.keep_declared(manifest["files"], [])
+                files, _ = install.narrow_to_cores(manifest, names, files, omitted)
+                self.assertEqual(files[0]["sha1"][0], letter)
+
+    def test_the_declared_event_judges_the_copy_the_platform_checks(self):
+        """melonds' copy is served at the slot, the platform checks its own:
+        the event and a --declared-only check give the same answer."""
+        import hashlib
+
+        tmp = Path(tempfile.mkdtemp(dir=REPO_ROOT / "tmp"))
+        self.addCleanup(shutil.rmtree, tmp, True)
+        payloads = {letter: letter.encode() * 3 for letter in "abc"}
+        manifest = self.manifest()
+        entry = manifest["files"][0]
+
+        def real(copy: dict, letter: str) -> None:
+            copy.update(sha1=hashlib.sha1(payloads[letter]).hexdigest(),
+                        sha256=hashlib.sha256(payloads[letter]).hexdigest(), size=3)
+
+        real(entry, "a")
+        real(entry["slot_choices"][1], "a")
+        real(entry["declared_choices"][0], "c")
+        real(entry["declared_choices"][1], "b")
+        (tmp / "dsi_bios9.bin").write_bytes(payloads["c"])
+
+        def declared_event(files: list[dict]) -> dict:
+            _todo, fine, wrong = install.check_local(files, tmp)
+            with mock.patch.object(install.EVENTS, "emit") as emit:
+                install._emit_check("retroarch", manifest, tmp, files, fine, wrong, 0, 0)
+            return next(call.kwargs for call in emit.call_args_list
+                        if call.args[0] == "declared")
+
+        for names in (None, ["melonds"], ["desmume", "melonds"]):
+            with self.subTest(names=names):
+                served = manifest["files"]
+                own, _ = install.keep_declared(manifest["files"], [])
+                if names is not None:
+                    served, _ = install.narrow_to_cores(manifest, names)
+                    own, _ = install.narrow_to_cores(manifest, names, own, [])
+                self.assertEqual(declared_event(served), declared_event(own))
+                self.assertEqual(declared_event(own)["present"], 1)
+
     def test_the_boundary_checks_every_copy(self):
         install._validate_manifest(self.manifest(), "retroarch")
         bad_choices = (