Block conflicting extension write selectors

tests/1c/test_write_preflight_smoke.py

@@ -112,6 +112,22 @@ def test_public_extension_form_candidate_is_name_first_and_prefers_title() -> No
     }
 
 
+def test_different_extension_guid_changes_only_last_hex_digit() -> None:
+    assert (
+        smoke.different_extension_guid(
+            "extension:fb26cf42-7609-11f1-828f-005056b0d483"
+        )
+        == "fb26cf42-7609-11f1-828f-005056b0d480"
+    )
+    assert (
+        smoke.different_extension_guid(
+            "extension:fb26cf42-7609-11f1-828f-005056b0d480"
+        )
+        == "fb26cf42-7609-11f1-828f-005056b0d481"
+    )
+    assert smoke.different_extension_guid("base") is None
+
+
 def test_name_first_extension_form_preflight_rejects_base_gate_layer(monkeypatch) -> None:
     def fake_rpc_call(
         _endpoint_url: str,