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/services/types.go

@@ -25,4 +25,11 @@ func RegisterPluginWithPorts(name string, factory func() Plugin, ports []int) {
 	}, ports)
 }
 
+// RegisterUDPPluginWithPorts 注册UDP协议插件
+func RegisterUDPPluginWithPorts(name string, factory func() Plugin, ports []int) {
+	plugins.RegisterUDPWithPorts(name, func() plugins.Plugin {
+		return factory()
+	}, ports)
+}
+
 var GenerateCredentials = plugins.GenerateCredentials