Review follow-up: the close listener is a CloseEvent by construction (code is always a number), so the
`as { code?: unknown }` cast and the 'unknown' fallback guarded nothing. Renderer 'error' events carry no
message, and every consumer of this client is renderer-side, so the `: detail` suffix was never produced —
the class alone is the detail. Test: the fake socket loses its unused OPEN/static-last bookkeeping and the
prefix test asserts the real contract (prefix kept, not equal to the bare message) instead of prose.
6338e988bd