Initial commit: MikroTik btest server & client in Rust

Full reimplementation of the MikroTik Bandwidth Test protocol:
- Server mode: accepts connections from MikroTik devices on port 2000
- Client mode: connects to MikroTik btest servers
- TCP and UDP protocols with bidirectional support
- MD5 challenge-response authentication
- Dynamic speed adjustment (1.5x algorithm)
- Status exchange matching original C pselect() behavior
- Docker support with multi-stage build

Tested against MikroTik RouterOS achieving:
- 1.05 Gbps server RX (single connection)
- 530 Mbps client TCP download
- 840 Mbps client TCP upload
- 433 Mbps client UDP download

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

scripts/test-local.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+# Local loopback tests - run server and client against each other
+set -euo pipefail
+
+BTEST="cargo run --release --"
+PORT=2000
+
+echo "=== Building release binary ==="
+cargo build --release
+
+BTEST="./target/release/btest"
+
+cleanup() {
+    echo "Stopping server..."
+    kill $SERVER_PID 2>/dev/null || true
+    wait $SERVER_PID 2>/dev/null || true
+}
+
+echo ""
+echo "=== Starting btest server on port $PORT ==="
+$BTEST -s -P $PORT -v &
+SERVER_PID=$!
+trap cleanup EXIT
+sleep 1
+
+TIMEOUT_CMD="timeout"
+if ! command -v timeout &>/dev/null; then
+    if command -v gtimeout &>/dev/null; then
+        TIMEOUT_CMD="gtimeout"
+    else
+        # Fallback: background + sleep + kill
+        TIMEOUT_CMD=""
+    fi
+fi
+
+run_test() {
+    local desc="$1"
+    shift
+    echo ""
+    echo "--- Test: $desc ---"
+    if [ -n "$TIMEOUT_CMD" ]; then
+        $TIMEOUT_CMD 5 $BTEST "$@" || true
+    else
+        $BTEST "$@" &
+        local pid=$!
+        sleep 5
+        kill $pid 2>/dev/null || true
+        wait $pid 2>/dev/null || true
+    fi
+    echo "--- Done: $desc ---"
+    sleep 1
+}
+
+run_test "TCP Download (RX)" -c 127.0.0.1 -P $PORT -r
+run_test "TCP Upload (TX)" -c 127.0.0.1 -P $PORT -t
+run_test "TCP Bidirectional" -c 127.0.0.1 -P $PORT -t -r
+run_test "TCP Download 100Mbps limited" -c 127.0.0.1 -P $PORT -r -b 100M
+run_test "UDP Download" -c 127.0.0.1 -P $PORT -r -u
+run_test "UDP Upload" -c 127.0.0.1 -P $PORT -t -u
+run_test "UDP Bidirectional" -c 127.0.0.1 -P $PORT -t -r -u
+
+echo ""
+echo "=== All local tests completed ==="