rewriteRequest shared routing

Chose exported async rewriteRequest(Request, gatewayBase, headers, observer) shared with installFetchInterceptor() over separate routing implementations because bypass, headers, paths, and stream semantics must not drift. The installer must preserve caller shape: Request inputs remain Requests; string/URL inputs remain (url, init) so body and init.headers consumers keep working. It may reuse a pre-parsed gateway URL; the public API remains string-based.