fix(icons): faviconV2 加入 drop_404_icon=true 参数避免404占位图以触发回退

- helpers：faviconV2Url / faviconFallbackUrl 统一追加 drop_404_icon=true
- runtime：新增站点的 faviconV2 com/cn URL 同步追加该参数
- docs：更新模板与 helper 文档示例
- test：新增用例防止参数回归

test/favicon-v2-drop-404-icon.node-test.js

@@ -0,0 +1,32 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const { faviconV2Url, faviconFallbackUrl } = require('../src/helpers/utils');
+
+test('faviconV2：应追加 drop_404_icon=true 以避免返回占位图', () => {
+  const optionsCom = { data: { root: { icons: { region: 'com' } } } };
+  const optionsCn = { data: { root: { icons: { region: 'cn' } } } };
+
+  const url = 'https://example.com';
+
+  const com = faviconV2Url(url, optionsCom);
+  const cn = faviconV2Url(url, optionsCn);
+  const fallbackCom = faviconFallbackUrl(url, optionsCom);
+  const fallbackCn = faviconFallbackUrl(url, optionsCn);
+
+  for (const out of [com, cn, fallbackCom, fallbackCn]) {
+    assert.ok(out.includes('drop_404_icon=true'), '生成的 URL 应包含 drop_404_icon=true');
+  }
+});
+
+test('运行时新增站点：faviconV2 URL 也应包含 drop_404_icon=true', () => {
+  const repoRoot = path.resolve(__dirname, '..');
+  const runtimePath = path.join(repoRoot, 'src', 'runtime', 'menav', 'addElement.js');
+  const content = fs.readFileSync(runtimePath, 'utf8');
+  assert.ok(
+    content.includes('drop_404_icon=true'),
+    'src/runtime/menav/addElement.js 应追加 drop_404_icon=true'
+  );
+});