core: carry credentials through the probe and its auth handshake

libcurl with CURLAUTH_ANY answers a 401/407 by resending the request with
an Authorization header. Two spots in net/ cut that short:

- http_client's header callback delivered the response head exactly once
  and latched `head_delivered`, so after an auth challenge the caller only
  ever saw the 401 — never the 2xx of the authenticated resend. Reset the
  latch when a fresh status line follows a delivered 401/407 (redirects
  never reach that path — their head is suppressed).

- the prober's head callbacks return DataAction::abort to skip the body,
  which also aborts the transfer mid-handshake. Return `proceed` for a
  401/407 when credentials were supplied, so curl's resend can run; the
  real status lands on the next header block.

Also give ProbeRequest an `auth` field (default scheme == none) and pass
it through base_request(), so a re-probe after a 401 can present the
credentials the user just entered. No behaviour change when no auth is
configured.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HPPSGhiArbvQgwC2DNiURS

core/include/vdm/net/probe.hpp

@@ -35,6 +35,7 @@ struct ProbeRequest {
     std::string user_agent;
     std::string referrer;
     ProxyConfig proxy;
+    AuthConfig auth;  // credentials for a re-probe after a 401 (leave scheme == none otherwise)
 
     long connect_timeout_ms = 15000;
     long overall_timeout_ms = 25000;  // download.probe deadline is 30 s