Fix multi-connection: don't connect() UDP socket when conn_count > 1

Root cause found via pcap analysis: MikroTik with connection-count=N
sends UDP from N different source ports (2257, 2258, 2259, ...) all
to our single server port 2001. A connect()'d UDP socket only accepts
packets from the one connected address, silently dropping ~75% of
traffic with conn_count=4.

Fix: when tcp_conn_count > 0, leave the UDP socket unconnected and
use send_to()/recv_from() instead of send()/recv(). This accepts
packets from all MikroTik source ports.

This bug also exists in the original C btest-opensource.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/debug-capture.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# Capture btest traffic for debugging multi-connection issues.
+#
+# Usage:
+#   # Terminal 1: Start capture
+#   sudo ./scripts/debug-capture.sh capture <interface> [mikrotik_ip]
+#
+#   # Terminal 2: Run server or client
+#   ./target/release/btest -s -a admin -p password -vv
+#
+#   # Terminal 1: Stop with Ctrl+C, then analyze
+#   ./scripts/debug-capture.sh analyze
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+CMD="${1:?Usage: $0 <capture|analyze> [interface] [mikrotik_ip]}"
+
+PCAP_FILE="dist/btest-debug.pcap"
+mkdir -p dist
+
+case "$CMD" in
+    capture)
+        IFACE="${2:?Specify interface (e.g., en0, eth0)}"
+        MK_IP="${3:-}"
+
+        FILTER="port 2000 or portrange 2001-2100 or portrange 2257-2356"
+        if [[ -n "$MK_IP" ]]; then
+            FILTER="host $MK_IP and ($FILTER)"
+        fi
+
+        echo "Capturing btest traffic on $IFACE..."
+        echo "Filter: $FILTER"
+        echo "Output: $PCAP_FILE"
+        echo "Press Ctrl+C to stop"
+        echo ""
+        tcpdump -i "$IFACE" -w "$PCAP_FILE" -s 128 "$FILTER"
+        ;;
+
+    analyze)
+        if [[ ! -f "$PCAP_FILE" ]]; then
+            echo "No capture file found at $PCAP_FILE"
+            echo "Run: sudo $0 capture <interface> first"
+            exit 1
+        fi
+
+        echo "=== TCP Control Channel (port 2000) ==="
+        echo ""
+        echo "--- Connection summary ---"
+        tcpdump -r "$PCAP_FILE" -n 'tcp port 2000 and (tcp[tcpflags] & tcp-syn != 0)' 2>/dev/null | head -20
+        echo ""
+
+        echo "--- All TCP control data (first 64 bytes of payload) ---"
+        tcpdump -r "$PCAP_FILE" -n -X 'tcp port 2000 and tcp[tcpflags] & tcp-push != 0' 2>/dev/null | head -100
+        echo ""
+
+        echo "=== UDP Data Ports ==="
+        echo ""
+        echo "--- UDP port usage ---"
+        tcpdump -r "$PCAP_FILE" -n 'udp' 2>/dev/null | awk '{print $3, $5}' | sort | uniq -c | sort -rn | head -20
+        echo ""
+
+        echo "--- Timing of first packets per connection ---"
+        tcpdump -r "$PCAP_FILE" -n -tt 'tcp port 2000 and (tcp[tcpflags] & tcp-syn != 0)' 2>/dev/null | head -20
+        echo ""
+
+        echo "Full capture at: $PCAP_FILE"
+        echo "Open in Wireshark: wireshark $PCAP_FILE"
+        ;;
+
+    *)
+        echo "Usage: $0 <capture|analyze> [interface] [mikrotik_ip]"
+        exit 1
+        ;;
+esac