fix(net): 修复 40250 进入游戏时的收包错位

- GC_ITEM_DEL(20) 按 40250 旧结构为 42 字节，GC_REFINE_INFORMATION(95) 为 59 字节
- 公会标记连接接受 GC_MARK_DIFF_DATA(101)，按 1 字节消费
- GUILD_SUBHEADER_GC_SKILL_INFO 服务器声明 22 字节但实际只写 21 字节，
  按实际长度分帧，修复 "unknown GC header 200 (last: 75,20)" 断线
- 会话测试的 quest info 包改为 40250 按 flag 定长的格式，新增对应分帧测试

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ft3kXpNdLbKEfg5uDLfqVF

extension/tests/net_classic_mark_test.cpp

@@ -59,6 +59,12 @@ static void test_mark_download() {
 	assert(request.size() == sizeof(CGMarkIDXList));
 	assert(request[0] == HDR_CG_MARK_IDXLIST);
 
+	// GuildMarkDownloader.cpp:172/201 frames HEADER_GC_MARK_DIFF_DATA (101) as a
+	// bare header byte and ignores it; the next mark frame must still parse.
+	const uint8_t diff = 101;
+	client.stream().feed(&diff, 1);
+	assert(client.stream().last_error().empty());
+
 	// One guild maps to image zero, position one. Feed the whole-size frame in
 	// two chunks to exercise the custom raw framing path.
 	std::vector<uint8_t> idx(sizeof(GCMarkIDXList) + 4);