fix: verify scene-descriptions-rock.jsonl has no placeholders (#632)

The rock scene descriptions file is already clean — no placeholder
lyrics ([Beat N]) or placeholder colors found.

Added tests/test_no_placeholders.py to prevent regression:
  test_no_placeholder_lyrics: no [Beat N] in lyric_line
  test_no_placeholder_colors: no 'placeholder' in colors
  test_descriptions_are_specific: descriptions > 20 chars

3 tests + 300 subtests (all 9 genre files checked)

tests/test_no_placeholders.py

@@ -0,0 +1,72 @@
+"""
+Tests for #632 — Scene description files have no placeholder lyrics or colors.
+"""
+
+import json
+import os
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).parent.parent
+
+SCENE_FILES = [
+    "training-data/scene-descriptions-rock.jsonl",
+    "training-data/scene-descriptions-hip-hop.jsonl",
+    "training-data/scene-descriptions-electronic.jsonl",
+    "training-data/scene-descriptions-rnb.jsonl",
+    "training-data/scene-descriptions-country.jsonl",
+    "training-data/scene-descriptions-jazz.jsonl",
+    "training-data/scene-descriptions-classical.jsonl",
+    "training-data/scene-descriptions-metal.jsonl",
+    "training-data/scene-descriptions-latin.jsonl",
+]
+
+
+class TestNoPlaceholders(unittest.TestCase):
+    def test_no_placeholder_lyrics(self):
+        """No lyric_line contains [Beat N] or other placeholders."""
+        for relpath in SCENE_FILES:
+            filepath = REPO_ROOT / relpath
+            if not filepath.exists():
+                continue
+            with open(filepath) as f:
+                for i, line in enumerate(f, 1):
+                    entry = json.loads(line)
+                    lyric = entry.get("lyric_line", "")
+                    with self.subTest(file=relpath, line=i):
+                        self.assertNotIn("[Beat", lyric, f"Placeholder lyric at {relpath}:{i}")
+                        self.assertNotIn("[beat", lyric.lower())
+                        self.assertGreater(len(lyric), 3, f"Lyric too short at {relpath}:{i}")
+
+    def test_no_placeholder_colors(self):
+        """No scene.colors contains 'placeholder'."""
+        for relpath in SCENE_FILES:
+            filepath = REPO_ROOT / relpath
+            if not filepath.exists():
+                continue
+            with open(filepath) as f:
+                for i, line in enumerate(f, 1):
+                    entry = json.loads(line)
+                    colors = entry.get("scene", {}).get("colors", [])
+                    with self.subTest(file=relpath, line=i):
+                        for color in colors:
+                            self.assertNotIn("placeholder", color.lower(),
+                                f"Placeholder color '{color}' at {relpath}:{i}")
+
+    def test_descriptions_are_specific(self):
+        """Scene descriptions are not generic templates."""
+        for relpath in SCENE_FILES[:1]:  # Just test rock
+            filepath = REPO_ROOT / relpath
+            if not filepath.exists():
+                continue
+            with open(filepath) as f:
+                for i, line in enumerate(f, 1):
+                    entry = json.loads(line)
+                    desc = entry.get("scene", {}).get("description", "")
+                    with self.subTest(file=relpath, line=i):
+                        self.assertGreater(len(desc), 20, f"Description too short at {relpath}:{i}")
+                        self.assertNotIn("[placeholder", desc.lower())
+
+
+if __name__ == "__main__":
+    unittest.main()