test(v2): define protocol and clean-break contracts

tests/contract/test_no_v1.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+from pathlib import Path
+import subprocess
+import sys
+
+
+ROOT = Path(__file__).parents[2]
+
+
+def test_v1_executable_packages_are_absent() -> None:
+    assert not (ROOT / "backend" / "app").exists()
+    assert not (ROOT / "backend" / "backup").exists()
+    assert not (ROOT / "backend" / "tests").exists()
+    assert not (ROOT / "frontend" / "src" / "api" / "client.ts").exists()
+
+
+def test_v1_reference_tag_exists() -> None:
+    result = subprocess.run(
+        ["git", "tag", "--list", "v1-reference-2026-07-27"],
+        cwd=ROOT,
+        capture_output=True,
+        check=False,
+        text=True,
+    )
+    assert result.returncode == 0
+    assert result.stdout.strip() == "v1-reference-2026-07-27"
+
+
+def test_forbidden_v1_scanner_accepts_runtime_tree() -> None:
+    result = subprocess.run(
+        [sys.executable, "tools/forbidden_v1_scan.py", "."],
+        cwd=ROOT,
+        capture_output=True,
+        check=False,
+        text=True,
+    )
+    assert result.returncode == 0, result.stdout + result.stderr