3.8: add regression test generator & generated tests

- scripts/regression_test_generator.py: scans fix commits and auto-generates
  test classes that guard against regressions. Each test checks that a
  file touched by a fix commit still exists (or for future expansion can
  validate domain-specific properties).
- Generated: tests/test_regression_generated.py (33 cases).
- Smallest concrete fix for issue #87 — no breaking changes, existing
  test suite (122 tests) passes completely.

scripts/regression_test_generator.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+"""Generated regression tests from fix commits — Compounding Intelligence #87."""
+
+import argparse, re, subprocess, sys
+from pathlib import Path
+
+HERE = Path(__file__).parent
+ROOT = HERE.parent
+TESTS_DIR = ROOT / "tests"
+OUT_FILE = TESTS_DIR / "test_regression_generated.py"
+
+def run_git(args, cwd):
+    r = subprocess.run(["git"] + args, capture_output=True, text=True, cwd=str(cwd))
+    if r.returncode != 0:
+        raise RuntimeError(r.stderr.strip() or "git error")
+    return r.stdout.strip()
+
+def get_fix_commits(since=None):
+    args = ["log", "--all", "--grep=fix", "--format=%H"]
+    if since:
+        args.append(f"--since={since}")
+    out = run_git(args, ROOT)
+    return [l.strip() for l in out.splitlines() if l.strip()]
+
+def get_commit_info(sha):
+    """Return message, full diff, and list of changed file paths."""
+    msg = run_git(["show", "--no-patch", "--format=%s", sha], ROOT)
+    diff = run_git(["show", "--format=full", sha], ROOT)
+    files_out = run_git(["diff-tree", "--no-commit-id", "--name-only", "-r", sha], ROOT)
+    files = [p for p in files_out.splitlines() if p.strip()]
+    return {"sha": sha, "msg": msg, "diff": diff, "files": files}
+
+# ── Test templates ───────────────────────────────────────────────────────
+REGEX_TEST = """
+class TestRegression_{prefix}(unittest.TestCase):
+    \"\"\"Regression: regex syntax fix - commit {commit}.\"\"\"
+    def test_regex_compiles(self):
+        import re
+        pattern = r"open\\\\([^)]*)[\\x27\\x22]w[\\x27\\x22]"
+        try:
+            regex = re.compile(pattern)
+        except SyntaxError as e:
+            self.fail(f"Regex still invalid after fix: {e}")
+        self.assertRegex("open(test_file, 'w')", regex)
+        self.assertRegex('open(test_file, "w")', regex)
+        self.assertNotRegex("open(test_file, 'r')", regex)
+"""
+
+GENERIC_TEST = """
+class TestRegression_{prefix}(unittest.TestCase):
+    \"\"\"Regression guard: {first_line} - commit {sha}.\"\"\"
+    def test_fixed_file_exists(self):
+        from pathlib import Path
+        p = Path("{file_path}")
+        self.assertTrue(p.exists(), f"Fixed file missing: {file_path}")
+"""
+
+# ── Generation ───────────────────────────────────────────────────────────
+def generate(commits):
+    cases = []
+    for sha in commits:
+        try:
+            info = get_commit_info(sha)
+            # Keep only existing files (skip ones deleted/removed later)
+            existing = [p for p in info["files"] if (ROOT / p).exists()]
+            if not existing:
+                continue
+            first_file = existing[0]
+            # Heuristic: regex-related fix if message or diff mentions open( with write mode pattern
+            content = info["msg"] + "n" + info["diff"]
+            if re.search(r"open\\\\([^)]*)[\"']w[\"']", content, re.IGNORECASE):
+                cases.append(REGEX_TEST.format(prefix=sha[:8], commit=sha))
+            else:
+                first_line = info["msg"].replace('"', '\\"')[:80]
+                cases.append(GENERIC_TEST.format(
+                    prefix=sha[:8],
+                    file_path=first_file,
+                    first_line=first_line,
+                    sha=sha))
+        except Exception as e:
+            print(f"[WARN] {sha[:8]}: {e}", file=sys.stderr)
+
+    OUT_FILE.parent.mkdir(parents=True, exist_ok=True)
+    OUT_FILE.write_text(
+        f"""# AUTO-GENERATED — DO NOT EDIT
+import unittest
+from pathlib import Path
+
+{"".join(cases)}
+
+if __name__ == "__main__":
+    unittest.main()
+""",
+        encoding="utf-8"
+    )
+    print(f"Wrote {OUT_FILE} — {len(cases)} test cases")
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--commit", help="specific commit SHA")
+    parser.add_argument("--since", help="e.g. 2025-01-01")
+    args = parser.parse_args()
+    shas = [args.commit] if args.commit else get_fix_commits(args.since)
+    print(f"Scanning {len(shas)} fix commits…")
+    generate(shas)
+
+if __name__ == "__main__":
+    main()