fix: refuse a platform list where one is taken

generate_pack split --platform on commas to validate it, then used
the whole string as one platform and crashed. Every script takes a
single platform; a list is refused with a message pointing at --all.

scripts/common.py

@@ -424,6 +424,13 @@ def require_platform(name: str, platforms_dir: str = "platforms") -> None:
     A typo used to end in a traceback (verify, cross_reference) or in "no
     pack for batocra" after the whole build ran (generate_pack).
     """
+    if "," in name:
+        # generate_pack split the list to validate it, then used the whole
+        # string as one name and ended in a traceback.
+        raise SystemExit(
+            f"ERROR: --platform takes one platform, not '{name}'. "
+            "Run once per platform, or use --all."
+        )
     if os.path.exists(os.path.join(platforms_dir, f"{name}.yml")):
         return
     import difflib  # noqa: PLC0415

scripts/generate_pack.py

@@ -2917,8 +2917,7 @@ def _main(held: contextlib.ExitStack) -> None:
     args = parser.parse_args()
     packresolve.set_offline(bool(args.offline))
     if args.platform:
-        for name in args.platform.split(","):
-            require_platform(name.strip(), args.platforms_dir)
+        require_platform(args.platform, args.platforms_dir)
 
     _refuse_unapplied_flags(args, parser)
     if args.target:

tests/test_audit_regressions.py

@@ -375,6 +375,38 @@ class VerboseAppliesToJson(unittest.TestCase):
         self.assertNotIn('if d["status"] != Status.OK\n            ]', source)
 
 
+class OnePlatformPerFlag(unittest.TestCase):
+    """generate_pack validated --platform as a comma list, then used the whole
+    string as one name: `--platform bizhawk,misterfpga` passed and ended in a
+    FileNotFoundError traceback. Every script now says it takes one name."""
+
+    def test_a_list_is_refused_with_a_message(self):
+        import subprocess
+
+        runs = {
+            "generate_pack.py": ["--list-systems"],
+            "verify.py": [],
+            "cross_reference.py": [],
+        }
+        for script, extra in runs.items():
+            with self.subTest(script=script):
+                completed = subprocess.run(
+                    [sys.executable, f"scripts/{script}", "--platform",
+                     "bizhawk,misterfpga", *extra],
+                    capture_output=True, text=True, cwd=ROOT, timeout=600, check=False,
+                )
+                self.assertNotEqual(completed.returncode, 0)
+                self.assertIn("takes one platform", completed.stderr)
+                self.assertNotIn("Traceback", completed.stderr)
+
+    def test_unknown_names_still_get_a_suggestion(self):
+        from common import require_platform
+
+        with self.assertRaises(SystemExit) as caught:
+            require_platform("batocra", str(ROOT / "platforms"))
+        self.assertIn("batocera", str(caught.exception))
+
+
 class FaqRegressions(unittest.TestCase):
     """The FAQ states facts the code owns; these tie it back to the source.