Implemented the next 4h-plan phase: dual-run support + explicit cutover gate.

scripts/compare_rain_reports.py

@@ -16,6 +16,8 @@ def parse_args() -> argparse.Namespace:
 
 def load_json(path: str) -> dict[str, Any]:
     p = Path(path)
+    if not p.exists():
+        raise FileNotFoundError(path)
     with p.open("r", encoding="utf-8") as f:
         return json.load(f)
 
@@ -42,8 +44,25 @@ def delta_str(base: float | None, cand: float | None) -> str:
 
 def main() -> int:
     args = parse_args()
-    baseline = load_json(args.baseline)
-    candidate = load_json(args.candidate)
+    try:
+        baseline = load_json(args.baseline)
+    except FileNotFoundError:
+        print(f"error: baseline report not found: {args.baseline}")
+        model_dir = Path("models")
+        if model_dir.exists():
+            candidates = sorted(model_dir.glob("rain_model_report*.json"))
+            if candidates:
+                print("available report files:")
+                for c in candidates:
+                    print(f"  - {c}")
+        print("hint: provide an existing 1h report path, or train a new 1h report first.")
+        return 2
+
+    try:
+        candidate = load_json(args.candidate)
+    except FileNotFoundError:
+        print(f"error: candidate report not found: {args.candidate}")
+        return 2
 
     pairs = [
         ("precision", metric(baseline, "test_metrics", "precision"), metric(candidate, "test_metrics", "precision")),