e9a77f4188's `_RUNTIME_MAIN_API_KEY == old` guard had no test: reverting it left every kept test
green, though rotating under scoped_runtime_main then raises KeyError 'base_url' (or, with the
.get() default, overwrites the turn's mirrors with the scoped copy). Extend the existing aux-route
test with the scoped rotate step; red with the guard removed (mirror model becomes 'm') and red on
the pre-guard code (KeyError), green on HEAD.
Fold finding C15 #2 (gate 2ab toothless guard).
4c8d4148ca