$tx->req->headers->header(name => value) — the one piece of Mojo::Transaction surface an
->on('start', sub ($ua, $tx) {...}) handler (see LegacyUserAgent.on below) actually reaches
into across the legacy plugin corpus (always to set a static request header, never to read
anything back off $tx), so this is intentionally not a general Mojo::Transaction shim.
req: { headers: { header(name: string,value: string): void; }; }