R-896: the volume-persistence gate refuses a host that is not a scratch host (no containers on DooPlex)

check() refuses (HARNESS REFUSED, exit 3) before any docker call unless /opt/upg exists (the bench)
or FELHOM_SCRATCH_HOST=1. TestVenue red before the guard; new end-to-end decoy.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

scripts/test_check_volume_persistence.py

@@ -9,6 +9,7 @@ Run: python3 scripts/test_check_volume_persistence.py
 """
 import importlib.util
 import io
+import os
 import sys
 import tempfile
 import unittest
@@ -597,5 +598,64 @@ class TestEmptyBind(unittest.TestCase):
         self.assertFalse(any("R-805" in w for w in why), why)
 
 
+class TestVenue(unittest.TestCase):
+    """R-896: the REAL prober runs containers on the local Docker. On a host that is not a scratch host (DooPlex is
+    the production host) the gate refuses BEFORE any docker call — the consequence, not the helper: the prober is
+    never reached."""
+
+    def setUp(self):
+        self._env = os.environ.pop(cvp.SCRATCH_ENV, None)
+        self._markers = cvp.SCRATCH_MARKERS
+        self._real = cvp.docker_prober
+        self.calls = []
+
+    def tearDown(self):
+        if self._env is not None:
+            os.environ[cvp.SCRATCH_ENV] = self._env
+        else:
+            os.environ.pop(cvp.SCRATCH_ENV, None)
+        cvp.SCRATCH_MARKERS = self._markers
+
+    def _run(self):
+        def spy(*a, **kw):
+            self.calls.append(a)
+            raise AssertionError("the real prober was reached")
+        with tempfile.TemporaryDirectory() as td:
+            d = Path(td) / "templates" / "demo"
+            d.mkdir(parents=True)
+            (d / "docker-compose.yml").write_text("services:\n  s:\n    image: alpine:3.22\n")
+            (d / ".felhom.yml").write_text("slug: demo\n")
+            orig_self_test = cvp.self_test
+            cvp.self_test = lambda prober: spy()
+            try:
+                buf = io.StringIO()
+                with redirect_stdout(buf):
+                    rc = cvp.check(Path(td))   # the default prober = the real one
+            finally:
+                cvp.self_test = orig_self_test
+        return rc, buf.getvalue()
+
+    def test_no_marker_refuses_before_any_docker_call(self):
+        with tempfile.TemporaryDirectory() as nowhere:
+            cvp.SCRATCH_MARKERS = (Path(nowhere) / "absent",)
+            rc, out = self._run()
+        self.assertEqual(rc, cvp.HARNESS_REFUSED, out)
+        self.assertIn("not a scratch host", out)
+        self.assertEqual(self.calls, [])
+
+    def test_marker_directory_lets_it_run(self):
+        with tempfile.TemporaryDirectory() as here:
+            cvp.SCRATCH_MARKERS = (Path(here),)
+            with self.assertRaises(AssertionError):   # it went on to the self-test (the spy)
+                self._run()
+
+    def test_explicit_env_lets_it_run(self):
+        with tempfile.TemporaryDirectory() as nowhere:
+            cvp.SCRATCH_MARKERS = (Path(nowhere) / "absent",)
+            os.environ[cvp.SCRATCH_ENV] = "1"
+            with self.assertRaises(AssertionError):
+                self._run()
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)