test: update stale drift guards (login-page A mark, 25 exempt ports)

Both predate this session's changes and were masked by the release
gate's cargo-test-weekly compile timeout:
- login_page_sources_its_art_from_the_gate still asserted the retired
  wordmark (logo-archipelago.svg); the login page ships the sidebar A
  mark (favico-black-v2.svg) since the 2026-08-05 rework.
- unauthenticated_ports_are_all_accounted_for lagged at 17; the
  v1.7.123 port-policy round grew the rationale-carrying exempt set
  to 25 (reviewed and enumerated in the test comment).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/appgate/mod.rs

@@ -932,7 +932,10 @@ mod tests {
             );
         }
         // Every referenced asset must be one the gate will actually serve.
-        assert!(read_ui_asset("logo-archipelago.svg").is_some() || cfg!(not(debug_assertions)));
+        // The logo is the sidebar A mark (favico-black-v2.svg) since the
+        // 2026-08-05 login-page rework — the old wordmark is off the
+        // allowlist on purpose.
+        assert!(read_ui_asset("favico-black-v2.svg").is_some() || cfg!(not(debug_assertions)));
     }
 
     /// The allowlist is the whole security boundary for asset serving: the