Dedupe the QUIC key-and-dial recipe across mux d and mux a

open   by a73x

Three copies: main.zig openAdminQuic, main.zig start --quic, muxa.zig openQuicConn. All three resolve the key by the same rule (--key, MUX_KEY_FILE, default path), parse HOST[:PORT], connect and pump to ready under a deadline. Only the refusal wording differs (prose vs muxa's JSON). Lift the shared part into quic.zig or a client-side helper that returns a typed refusal the caller words. Third copy means refactor.