fix: generate_truth refuses a target it lacks

--platform bizhawk --target switch printed skipped, then Done., and
exited 0 with no model written. generate_truth now resolves targets
through build_target_cores_cache, as verify and generate_pack do: a
named platform without the target is refused with exit 1, --all
leaves such platforms out and says so.

scripts/generate_truth.py

@@ -14,13 +14,13 @@ import sys
 
 sys.path.insert(0, os.path.dirname(__file__))
 from common import (
+    build_target_cores_cache,
     platform_arg,
     list_registered_platforms,
     load_data_dir_registry,
     load_database,
     load_emulator_profiles,
     load_platform_config,
-    load_target_config,
     require_directory,
     require_platform,
     require_yaml,
@@ -97,6 +97,19 @@ def main(argv: list[str] | None = None) -> None:
         require_platform(args.platform, args.platforms_dir)
         platforms = [args.platform]
 
+    # A named platform without that target is refused, as verify and
+    # generate_pack refuse it: skipped, it printed Done. and exited 0 with
+    # no model written. --all leaves such platforms out and says so.
+    target_cores_cache: dict[str, set[str] | None] = {}
+    if args.target:
+        try:
+            target_cores_cache, platforms = build_target_cores_cache(
+                platforms, args.target, args.platforms_dir, is_all=args.all
+            )
+        except (FileNotFoundError, ValueError) as exc:
+            print(f"ERROR: {exc}", file=sys.stderr)
+            sys.exit(1)
+
     # A target-narrowed model is a different artifact: written under the
     # platform's own name it overwrote the full one, and diff_truth then
     # compared a narrowed model against the whole scrape. Its directory is
@@ -109,17 +122,7 @@ def main(argv: list[str] | None = None) -> None:
                 output_dir, [name], args.target, args.platforms_dir
             )
         os.makedirs(output_dir, exist_ok=True)
-        target_cores: set[str] | None = None
-        if args.target:
-            try:
-                target_cores = load_target_config(
-                    name,
-                    args.target,
-                    args.platforms_dir,
-                )
-            except FileNotFoundError:
-                print(f"  {name}: no target config, skipped")
-                continue
+        target_cores = target_cores_cache.get(name)
 
         # Load platform config (with inheritance) and registry entry
         try:

tests/test_truth_diff.py

@@ -60,6 +60,22 @@ class ATargetedModelIsItsOwnArtifact(unittest.TestCase):
             )
 
 
+    def test_a_named_platform_without_the_target_is_refused(self):
+        """bizhawk --target switch printed "skipped", then Done., exit 0,
+        and wrote no model: the diff after it failed instead."""
+        repo = pathlib.Path(__file__).resolve().parent.parent
+        with tempfile.TemporaryDirectory(dir=str(repo / "tmp")) as directory:
+            proc = subprocess.run(
+                [sys.executable, "scripts/generate_truth.py", "--platform", "bizhawk",
+                 "--target", "switch", "--output-dir", directory],
+                capture_output=True, check=False, text=True, cwd=str(repo), timeout=400,
+            )
+            self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
+            self.assertIn("bizhawk has no hardware targets", proc.stderr)
+            self.assertNotIn("Done.", proc.stdout)
+            self.assertEqual(list(pathlib.Path(directory).iterdir()), [])
+
+
 class ADiffThatComparedNothingFails(unittest.TestCase):
     """diff_truth --all skipped every platform without a model and exited 0."""