fix: write one truth model per target, not alias

generate_truth named a narrowed model's directory after the target
as typed, so three aliases of one target wrote three copies, and
the pipeline rebuilt the slug by hand to diff it. One function now
names the directory from the canonical target for both.

tests/test_target_names.py

@@ -107,6 +107,31 @@ class EachPlatformNamesTheTargetItsOwnWay(unittest.TestCase):
         self.assertNotIn("args.target = canonical_target_name(", source)
 
 
+class OneTruthModelPerTarget(unittest.TestCase):
+    """generate_truth slugged the typed name: --target nx, switch and
+    nintendo-switch wrote three copies of one model, and the pipeline's diff
+    rebuilt the same slug by hand to find it."""
+
+    def test_aliases_share_a_directory(self):
+        from common import list_registered_platforms, truth_target_dir  # noqa: PLC0415
+
+        platforms = list_registered_platforms(
+            str(REPO_ROOT / "platforms"), include_archived=True
+        )
+        dirs = {
+            truth_target_dir("truth", platforms, alias, str(REPO_ROOT / "platforms"))
+            for alias in ("nx", "switch", "nintendo-switch")
+        }
+        self.assertEqual(dirs, {str(Path("truth") / "nintendo-switch")})
+
+    def test_writer_and_reader_call_one_function(self):
+        for module in ("generate_truth.py", "pipeline.py"):
+            source = (REPO_ROOT / "scripts" / module).read_text()
+            with self.subTest(module=module):
+                self.assertIn("truth_target_dir(", source)
+                self.assertNotIn('re.sub(\n            r"[^a-z0-9]+"', source)
+
+
 class TheListingCountsWhatTheFilterUses(unittest.TestCase):
     """--list-targets showed Batocera x86_64 with 198 cores; --target x86_64
     filters on 187 once _overrides.yml removes machine variants and launchers."""