feat(stress): add stress test loop

Add stress.lua that auto-detects or accepts generator and brute solution
files, compiles C++ if needed, and launches scripts/stress.py in a
terminal buffer with session save/restore and cleanup autocmds.

Add scripts/stress.py as a standalone loop that runs generator → brute →
candidate, comparing outputs and exiting on the first mismatch.

scripts/stress.py

@@ -0,0 +1,107 @@
+#!/usr/bin/env python3
+import subprocess
+import sys
+
+
+def main() -> None:
+    argv = sys.argv[1:]
+    max_iterations = 1000
+    timeout = 10
+
+    positional: list[str] = []
+    i = 0
+    while i < len(argv):
+        if argv[i] == "--max-iterations" and i + 1 < len(argv):
+            max_iterations = int(argv[i + 1])
+            i += 2
+        elif argv[i] == "--timeout" and i + 1 < len(argv):
+            timeout = int(argv[i + 1])
+            i += 2
+        else:
+            positional.append(argv[i])
+            i += 1
+
+    if len(positional) != 3:
+        print(
+            "Usage: stress.py <generator> <brute> <candidate> "
+            "[--max-iterations N] [--timeout S]",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+    generator, brute, candidate = positional
+
+    for iteration in range(1, max_iterations + 1):
+        try:
+            gen_result = subprocess.run(
+                generator,
+                capture_output=True,
+                text=True,
+                shell=True,
+                timeout=timeout,
+            )
+        except subprocess.TimeoutExpired:
+            print(f"[stress] generator timed out on iteration {iteration}", file=sys.stderr)
+            sys.exit(1)
+
+        if gen_result.returncode != 0:
+            print(
+                f"[stress] generator failed on iteration {iteration} "
+                f"(exit code {gen_result.returncode})",
+                file=sys.stderr,
+            )
+            if gen_result.stderr:
+                print(gen_result.stderr, file=sys.stderr, end="")
+            sys.exit(1)
+
+        test_input = gen_result.stdout
+
+        try:
+            brute_result = subprocess.run(
+                brute,
+                input=test_input,
+                capture_output=True,
+                text=True,
+                shell=True,
+                timeout=timeout,
+            )
+        except subprocess.TimeoutExpired:
+            print(f"[stress] brute timed out on iteration {iteration}", file=sys.stderr)
+            print(f"\n--- input ---\n{test_input}", end="")
+            sys.exit(1)
+
+        try:
+            cand_result = subprocess.run(
+                candidate,
+                input=test_input,
+                capture_output=True,
+                text=True,
+                shell=True,
+                timeout=timeout,
+            )
+        except subprocess.TimeoutExpired:
+            print(f"[stress] candidate timed out on iteration {iteration}", file=sys.stderr)
+            print(f"\n--- input ---\n{test_input}", end="")
+            sys.exit(1)
+
+        brute_out = brute_result.stdout.strip()
+        cand_out = cand_result.stdout.strip()
+
+        if brute_out != cand_out:
+            print(f"[stress] mismatch on iteration {iteration}", file=sys.stderr)
+            print(f"\n--- input ---\n{test_input}", end="")
+            print(f"\n--- expected (brute) ---\n{brute_out}")
+            print(f"\n--- actual (candidate) ---\n{cand_out}")
+            sys.exit(1)
+
+        print(f"[stress] iteration {iteration} OK", file=sys.stderr)
+
+    print(
+        f"[stress] all {max_iterations} iterations passed",
+        file=sys.stderr,
+    )
+    sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()