feat: 初始实现 xmtdx —— 从零实现通达信 TCP A 股行情客户端

替代年久失修的 pytdx，修复已知 bug，保留未解字段供逆向分析。

主要内容：
- codec 层：get_price 变长编码、get_volume 自定义浮点、datetime/frame 解析
- transport 层：同步（socket）+ 异步（asyncio）双实现，共用命令层
- 命令层（11 条）：security_count/list/quotes/bars、minute_time（今日+历史）、
  transaction（当日+历史）、xdxr_info、finance_info、company_info
- 高层 API：TdxClient + AsyncTdxClient
- 单元测试 26 条，全部通过；真实服务器集成测试覆盖全部命令

修复 pytdx Bug #1–5：xdxr 循环读取错误位置、GBK 截断崩溃、
pre_close 误用 get_volume、逐笔/分时未解字段被丢弃

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_codec_datetime.py

@@ -0,0 +1,64 @@
+"""日期时间解码单元测试。"""
+
+import struct
+
+from xmtdx.codec.datetime_ import get_datetime, get_datetime_day, get_datetime_minute, get_time
+
+
+def _pack_minute(year: int, month: int, day: int, hour: int, minute: int) -> bytes:
+    zipday = ((year - 2004) << 11) | (month * 100 + day)
+    tminutes = hour * 60 + minute
+    return struct.pack("<HH", zipday, tminutes)
+
+
+def _pack_day(year: int, month: int, day: int) -> bytes:
+    return struct.pack("<I", year * 10000 + month * 100 + day)
+
+
+class TestGetDatetimeMinute:
+    def test_basic(self):
+        data = _pack_minute(2024, 4, 10, 14, 30)
+        y, mo, d, h, mi, pos = get_datetime_minute(data, 0)
+        assert (y, mo, d, h, mi) == (2024, 4, 10, 14, 30)
+        assert pos == 4
+
+    def test_open_time(self):
+        data = _pack_minute(2026, 1, 5, 9, 30)
+        y, mo, d, h, mi, pos = get_datetime_minute(data, 0)
+        assert h == 9 and mi == 30
+
+    def test_close_time(self):
+        data = _pack_minute(2026, 1, 5, 15, 0)
+        y, mo, d, h, mi, _ = get_datetime_minute(data, 0)
+        assert h == 15 and mi == 0
+
+
+class TestGetDatetimeDay:
+    def test_basic(self):
+        data = _pack_day(2026, 4, 10)
+        y, mo, d, pos = get_datetime_day(data, 0)
+        assert (y, mo, d) == (2026, 4, 10)
+        assert pos == 4
+
+
+class TestGetDatetime:
+    def test_minute_category(self):
+        data = _pack_minute(2026, 3, 15, 10, 0)
+        for cat in (0, 1, 2, 3, 7, 8):
+            y, mo, d, h, mi, _ = get_datetime(cat, data, 0)
+            assert h == 10 and mi == 0
+
+    def test_day_category(self):
+        data = _pack_day(2026, 3, 15)
+        for cat in (4, 5, 6, 9):
+            y, mo, d, h, mi, _ = get_datetime(cat, data, 0)
+            assert (y, mo, d) == (2026, 3, 15)
+            assert h == 15 and mi == 0
+
+
+class TestGetTime:
+    def test_basic(self):
+        data = struct.pack("<H", 14 * 60 + 30)  # 14:30
+        h, mi, pos = get_time(data, 0)
+        assert h == 14 and mi == 30
+        assert pos == 2