test(tooling): the repo-stats suite tells a sparse model from a padded one, and --out refuses a foreign CSV (improvise-1xfb.5.1)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

scripts/repo-stats

@@ -302,6 +302,12 @@ def main():
     ap.add_argument("-C", dest="repo", default=".", help="the repository (default .)")
     a = ap.parse_args()
     os.makedirs(a.out, exist_ok=True)
+    ours = {f"{n}.csv" for n in ["churn"] + ANALYSES}
+    foreign = sorted(f for f in os.listdir(a.out) if f.lower().endswith(".csv") and f not in ours)
+    if foreign:
+        print(f"repo-stats: {a.out} holds CSVs this script did not write, and "
+              f"`improvise import` would merge them: {', '.join(foreign)}", file=sys.stderr)
+        return 2
     try:
         r = report(a.repo, a.ref, a.since, a.min_revs, os.path.abspath(a.out))
     except ImproviseFailed as e: