docs: verify issue #600 visual scenes dataset

Add a verification note and regression test showing that
training/data/prompt-enhancement/visual-scenes-500.jsonl already
exists on main with 500 valid records.

docs/issue-600-verification.md

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+# Issue #600 Verification
+
+Status: already implemented on `main`.
+
+Issue: Prompt Enhancement: Visual Scenes — 500 Terse→Rich Pairs
+
+What is already present on `main`
+- `training/data/prompt-enhancement/visual-scenes-500.jsonl`
+- 500 JSONL records
+- every record includes `terse`, `rich`, and `domain`
+- every `domain` value is `visual scenes`
+- 500/500 full records are unique
+
+Evidence gathered from a fresh clone
+- `python3` validation against `training/data/prompt-enhancement/visual-scenes-500.jsonl` returned:
+  - `count = 500`
+  - `unique_records = 500`
+  - `unique_terse = 435`
+  - `domains = ['visual scenes']`
+  - `missing_keys = 0`
+- remote branch `fix/600` still exists from closed PR #731 (`feat: 500 visual scene prompt enhancement pairs (#600)`)
+- SHA-256 of `training/data/prompt-enhancement/visual-scenes-500.jsonl` on `origin/main` matches the same file on `fix/600`, which shows the requested dataset is already present on `main`
+
+Verification commands
+```bash
+python3 - <<'PY'
+import json
+from pathlib import Path
+path = Path('training/data/prompt-enhancement/visual-scenes-500.jsonl')
+records = [json.loads(line) for line in path.read_text().splitlines() if line.strip()]
+print('count', len(records))
+print('unique_records', len({json.dumps(r, sort_keys=True) for r in records}))
+print('unique_terse', len({r['terse'] for r in records}))
+print('domains', sorted({r.get('domain') for r in records}))
+print('missing_keys', sum(any(k not in r or not str(r[k]).strip() for k in ('terse', 'rich', 'domain')) for r in records))
+PY
+```
+
+Recommendation
+- Close issue #600 as already implemented on `main`.
+- This branch only adds a durable verification note and regression test so the zombie issue can be closed cleanly without regenerating duplicate training data.

tests/test_prompt_enhancement_visual_scenes.py

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+#!/usr/bin/env python3
+"""Verification tests for timmy-config issue #600."""
+
+import json
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+DATA_PATH = ROOT / "training" / "data" / "prompt-enhancement" / "visual-scenes-500.jsonl"
+DOC_PATH = ROOT / "docs" / "issue-600-verification.md"
+
+
+def load_records():
+    with DATA_PATH.open("r", encoding="utf-8") as handle:
+        return [json.loads(line) for line in handle if line.strip()]
+
+
+class TestVisualScenesPromptEnhancement(unittest.TestCase):
+    def test_dataset_exists(self):
+        self.assertTrue(DATA_PATH.exists(), f"Missing dataset: {DATA_PATH}")
+
+    def test_dataset_has_500_records(self):
+        self.assertEqual(len(load_records()), 500)
+
+    def test_schema_and_domain(self):
+        for idx, record in enumerate(load_records(), start=1):
+            for key in ("terse", "rich", "domain"):
+                self.assertIn(key, record, f"line {idx}: missing {key}")
+                self.assertIsInstance(record[key], str, f"line {idx}: {key} must be a string")
+                self.assertTrue(record[key].strip(), f"line {idx}: {key} is empty")
+            self.assertEqual(record["domain"], "visual scenes", f"line {idx}: wrong domain")
+
+    def test_records_are_unique(self):
+        records = load_records()
+        serialized = [json.dumps(record, sort_keys=True) for record in records]
+        self.assertEqual(len(serialized), len(set(serialized)), "duplicate full records found")
+
+    def test_verification_doc_exists_and_mentions_evidence(self):
+        self.assertTrue(DOC_PATH.exists(), f"Missing verification doc: {DOC_PATH}")
+        text = DOC_PATH.read_text(encoding="utf-8")
+        self.assertIn("Issue #600 Verification", text)
+        self.assertIn("training/data/prompt-enhancement/visual-scenes-500.jsonl", text)
+        self.assertIn("PR #731", text)
+        self.assertIn("already implemented on `main`", text)
+
+
+if __name__ == "__main__":
+    unittest.main()