feat(sns): sns-feed / sns-search 输出完整 media[] 字段 (#15)

在 #14 之上增量：把 sns-feed / sns-search 的 media_count 升级成完整 media[] 数组（含 url/thumb/key/token/md5/enc_idx/size + video_md5/duration），下游可直接做图片代理或离线渲染。

- 用 roxmltree（pure Rust，无 C 依赖）替代 regex 抽属性
- 字段命名对齐 artifacts 仓库 Python _parse_media，跨实现 diff 友好
- 14 个 sns 单测：作者新增 6 个 fixture（单图/三图/视频/纯文字/malformed/缺 totalSize）+ 已有 8 个保持
- 与之前 PR #14 的 --user XML fallback 修复 / SNS_MAX_LIMIT / SNS_MAX_SCAN / escape_like_pattern 完全兼容

Author: leeguooooo <guoli@zhihu.com>
Co-fixed-by: wx-cli-coder (rebase + 冲突解决 + 测试模块合并 + media_count 语义文档补充)

Cargo.lock

@@ -719,6 +719,12 @@ version = "0.8.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 
+[[package]]
+name = "roxmltree"
+version = "0.20.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c20b6793b5c2fa6553b250154b78d6d0db37e72700ae35fad9387a46f487c97"
+
 [[package]]
 name = "rusqlite"
 version = "0.31.0"
@@ -1315,6 +1321,7 @@ dependencies = [
  "md5",
  "pbkdf2",
  "regex",
+ "roxmltree",
  "rusqlite",
  "serde",
  "serde_json",