e1608: scan review fixes (EINTR in Scan::poll, cfg-based broadcast test, no per-tick copy)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/source/e1608/e1608_link.cpp

@@ -291,7 +291,10 @@ const std::vector<DeviceInfo>& Scan::poll() {
       socklen_t flen = sizeof from;
       const ssize_t n = ::recvfrom(s.get(), buf, sizeof buf, MSG_DONTWAIT,
                                    reinterpret_cast<sockaddr*>(&from), &flen);
-      if (n < 0) break;  // nothing more on this socket for now
+      if (n < 0) {
+        if (errno == EINTR) continue;
+        break;  // nothing more on this socket for now
+      }
       take_reply(buf, n, from, found_);
     }
   }