fix(scan): truncate shodan banners on rune boundaries (#326)

truncateBanner sliced by byte offset, so a multibyte utf-8 rune landing
across the limit was cut mid-sequence and emitted invalid utf-8. shodan
banners routinely carry non-ascii bytes, so this corrupted logged and
printed output. count runes instead so the cut lands on a boundary.

internal/scan/shodan.go

@@ -260,8 +260,8 @@ func truncateBanner(banner string, maxLen int) string {
 	banner = strings.ReplaceAll(banner, "\r\n", " ")
 	banner = strings.ReplaceAll(banner, "\n", " ")
 
-	if len(banner) > maxLen {
-		return banner[:maxLen] + "..."
+	if runes := []rune(banner); len(runes) > maxLen {
+		return string(runes[:maxLen]) + "..."
 	}
 	return banner
 }

internal/scan/shodan_test.go

@@ -18,6 +18,7 @@ import (
 	"net/http/httptest"
 	"testing"
 	"time"
+	"unicode/utf8"
 )
 
 func TestResolveHostname_IP(t *testing.T) {
@@ -50,6 +51,9 @@ func TestTruncateBanner(t *testing.T) {
 		{"this is a long banner", 10, "this is a ..."},
 		{"with\nnewlines\r\n", 50, "with newlines"},
 		{"  trimmed  ", 50, "trimmed"},
+		// truncation must land on a rune boundary, not split a multibyte rune
+		{"aaaé", 4, "aaaé"},
+		{"日本語テスト", 3, "日本語..."},
 	}
 
 	for _, tt := range tests {
@@ -57,6 +61,9 @@ func TestTruncateBanner(t *testing.T) {
 		if result != tt.expected {
 			t.Errorf("truncateBanner(%q, %d) = %q, want %q", tt.input, tt.maxLen, result, tt.expected)
 		}
+		if !utf8.ValidString(result) {
+			t.Errorf("truncateBanner(%q, %d) produced invalid UTF-8: %q", tt.input, tt.maxLen, result)
+		}
 	}
 }