feat: add probeDomain function for TCP reachability checks on discovered domains

src/DomainDiscovery.hpp

@@ -16,3 +16,7 @@ std::vector<DiscoveredDomain> discoverDomains(int maxDomains = 25);
 
 // Exposed for tests: parse a directory JSON payload into candidate domains.
 std::vector<DiscoveredDomain> parseDomainsFromJson(const std::string& json);
+
+// Probe a domain for TCP reachability on its httpPort (non-blocking, short timeout).
+// Returns true if the domain appears reachable (TCP connect succeeds or is in progress).
+bool probeDomain(const DiscoveredDomain& domain, int timeoutMs = 800);