test(project): the #1878 engine-stub tests call ast.unparse, which Python 3.8 lacks #1899

tests/project/test_engine_stubs_1878_unit.py::EngineProtocolTests::test_each_expkg_is_its_own_guarded_protocol and ::test_each_protocol_declares_what_its_engine_reads, added by #1882, fail on the Python 3.8 (KTLO) unit leg with AttributeError: module 'ast' has no attribute 'unparse'. Seen on main in Unit Tests run 39038572601 (d14a1b9), and on #1896.

_guarded_protocols (:93-96) compares ast.unparse(base) against 'Protocol'/'typing.Protocol', but ast.unparse only exists from Python 3.9.

KTLO legs read green even when tests fail (#1679), so this shows only in the annotations and the log.

Fix: match the base node directly, as ast.Name(id='Protocol') or ast.Attribute(value=ast.Name(id='typing'), attr='Protocol'), and check that no other tests/project file calls ast.unparse outside a version gate.