feat: SDK agent integration + UDP plugin framework + SNMP plugin

SDK enhancements for endpoint agent embedding:
- ScanWithController for pause/resume and live stats
- OnProgress callback for periodic progress reporting
- TaskID injection into every scan result
- ScanController with goroutine-safe pause/resume/stats
- Multi-target stats aggregation (race-free)

UDP plugin infrastructure:
- PluginTypeUDP registry with dedicated dispatch path
- DialUDP on ScanSession with rate limiting and packet counting
- UDP plugins bypass TCP port scan, probe targets directly
- FilterService excludes UDP plugins from TCP port matching

SNMP plugin (first UDP plugin):
- SNMPv2c GetRequest probe for sysDescr detection
- Community string brute force (public/private/community/etc)
- Pure stdlib implementation (encoding/asn1)
- Registered as safe default plugin on port 161/UDP

Tests: 95.7% SDK coverage, race-free, 50+ new test cases

plugins/init.go

@@ -92,6 +92,7 @@ const (
 	PluginTypeWeb     = "web"     // Web类型插件
 	PluginTypeLocal   = "local"   // 本地类型插件
 	PluginTypeService = "service" // 服务类型插件
+	PluginTypeUDP     = "udp"     // UDP协议插件，跳过TCP端口扫描
 )
 
 var (
@@ -122,6 +123,16 @@ func RegisterWithPorts(name string, factory func() Plugin, ports []int) {
 	RegisterWithTypes(name, factory, ports, []string{PluginTypeService})
 }
 
+// RegisterUDPWithPorts 注册UDP协议插件，跳过TCP端口扫描链路
+func RegisterUDPWithPorts(name string, factory func() Plugin, ports []int) {
+	RegisterWithTypes(name, factory, ports, []string{PluginTypeUDP})
+}
+
+// IsUDP 检查插件是否为UDP协议插件
+func IsUDP(pluginName string) bool {
+	return HasType(pluginName, PluginTypeUDP)
+}
+
 // RegisterWithTypes 注册带类型标签的插件
 func RegisterWithTypes(name string, factory func() Plugin, ports []int, types []string) {
 	RegisterWithOptions(name, factory, ports, types, !hasPluginType(types, PluginTypeLocal))