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

core/scanner.go

@@ -211,6 +211,12 @@ func ExecuteScanTasks(ctx context.Context, session *common.ScanSession, targets
 		default:
 		}
 
+		if session.PauseGate != nil {
+			if err := session.PauseGate(ctx); err != nil {
+				return
+			}
+		}
+
 		targetPort := target.Port
 
 		for _, pluginName := range pluginsToRun {
@@ -262,6 +268,12 @@ func executeScanTask(ctx context.Context, session *common.ScanSession, pluginNam
 	default:
 	}
 
+	if session.PauseGate != nil {
+		if err := session.PauseGate(ctx); err != nil {
+			return
+		}
+	}
+
 	// 长驻插件不进 WaitGroup，通过 ctx 管理生命周期
 	if longRunningPlugins[pluginName] {
 		ready := make(chan struct{}, 1)