UpstreamSnapshot routing and protocol authority

Use one immutable UpstreamSnapshot as the routing authority for URL, caller selection, protocol, provider, model, forwarded headers, and provider options. Separate mutable globals can describe different requests. At dispatch, check each supported protocol explicitly, including effectiveProtocol === "anthropic"; never use a catch-all Anthropic branch. Reject malformed or future unknown protocols before serialization or network activity, with regression coverage asserting zero upstream calls.