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

examples/embed-agent/main.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	fscan "github.com/shadow1ng/fscan/pkg/fscan"
+)
+
+func main() {
+	scanner := fscan.NewScanner(fscan.Config{
+		TaskID:       "task-001",
+		Timeout:      3 * time.Second,
+		Threads:      64,
+		DisablePing:  true,
+		DisableBrute: true,
+		Plugins:      []string{"ssh", "mysql", "redis", "ftp"},
+		OnProgress: func(p fscan.ScanProgress) {
+			fmt.Printf("[progress] %d/%d tasks, %d packets, paused=%v, elapsed=%s\n",
+				p.TasksCompleted, p.TasksTotal, p.Packets, p.Paused, p.Duration.Round(time.Millisecond))
+		},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	ctrl, reportCh, errCh := scanner.ScanWithController(ctx,
+		fscan.Target{Host: "127.0.0.1", Ports: []int{21, 22, 3306, 6379}},
+	)
+
+	// Simulate a pause command from control plane after 1 second.
+	go func() {
+		time.Sleep(1 * time.Second)
+		fmt.Println("[agent] pausing scan...")
+		ctrl.Pause()
+
+		// Check live stats while paused.
+		stats := ctrl.Stats()
+		fmt.Printf("[agent] stats while paused: completed=%d, packets=%d\n",
+			stats.TasksCompleted, stats.Packets)
+
+		time.Sleep(2 * time.Second)
+		fmt.Println("[agent] resuming scan...")
+		ctrl.Resume()
+	}()
+
+	report := <-reportCh
+	if err := <-errCh; err != nil {
+		fmt.Printf("[agent] scan error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("\n[agent] scan complete: %d results, %d vulns, %d services\n",
+		report.Summary.Total, report.Summary.Vulns, report.Summary.Services)
+
+	for _, result := range report.Results {
+		taskID, _ := result.DetailString("task_id")
+		fmt.Printf("  [%s] %s %s (task=%s)\n", result.Type, result.Target, result.Status, taskID)
+	}
+}