fix: 修复标题长度计算不准确的问题 (#410)

使用基于 UTF-16 编码的加权算法替换 go-runewidth，与小红书实际计算规则一致：
非ASCII字符算2字节，ASCII字符算1字节，向上取整除以2。

Closes #401

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

pkg/xhsutil/title.go

@@ -0,0 +1,17 @@
+package xhsutil
+
+import "unicode/utf16"
+
+// CalcTitleLength 计算小红书标题长度
+// 规则：非ASCII字符(中文、全角符号等)算2字节，ASCII字符算1字节，最终结果向上取整除以2
+func CalcTitleLength(s string) int {
+	byteLen := 0
+	for _, c := range utf16.Encode([]rune(s)) {
+		if c > 127 {
+			byteLen += 2
+		} else {
+			byteLen += 1
+		}
+	}
+	return (byteLen + 1) / 2
+}

pkg/xhsutil/title_test.go

@@ -0,0 +1,36 @@
+package xhsutil
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCalcTitleLength(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{name: "空字符串", input: "", want: 0},
+		{name: "纯中文", input: "你好世界", want: 4},
+		{name: "纯英文", input: "hello", want: 3},
+		{name: "纯数字", input: "12345", want: 3},
+		{name: "中英混合-OOTD穿搭分享", input: "OOTD穿搭分享", want: 6},
+		{name: "20个中文字刚好上限", input: "一二三四五六七八九十一二三四五六七八九十", want: 20},
+		{name: "40个英文字母等于20", input: "abcdefghijklmnopqrstuvwxyzabcdefghijklmn", want: 20},
+		{name: "单个emoji", input: "😀", want: 2},
+		{name: "中文加emoji", input: "今天好开心😀", want: 7},
+		{name: "奇数个英文字母向上取整", input: "a", want: 1},
+		{name: "两个英文字母", input: "ab", want: 1},
+		{name: "三个英文字母", input: "abc", want: 2},
+		{name: "全角符号", input: "！？", want: 2},
+		{name: "半角符号", input: "!?", want: 1},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, CalcTitleLength(tt.input))
+		})
+	}
+}