feat(sntp): 板端 SNTP 校时接入——coreSNTP 序列化器 + [sntp] 段 + 设置页（#73）

- vendored coreSNTP v2.0.0 序列化器（MIT，仅 serializer+LICENSE，零堆），
  48 字节报文编解码在库，节奏与准入判据在本方（RFC 4330 §10 调研口径）
- App/sync/time_sntp.c 挂 timeSyncTask 的 8888 事件循环，等待切片 ≤50 ms，
  不饿任务狗与 IWDG；懒建 UDP socket，代际变化/禁用/死错误码即关闭归还
- 防伪 = OriginTimestamp 逐位等于发出时 T1（含低 16 位随机小数位）；
  mode/stratum/闰秒指示/KoD 按 RFC 拒收；DENY·RSTR 换服务器、RATE 只退避
- 主备轮换 + 失败指数退避（5 s 起、60 s 封顶）+ 首轮 1~5 分钟随机错峰；
  成功恢复 poll_min 周期（1~1440 分，出厂默认 60，默认不启用不发包）
- 钟面 = 本地时间：UTC+8 固定偏移落 sys_clock_set_unix（§1.1 时基裁定，
  第三个写入口，装置仍不存时区）；|offset|<2 s 只记日志不步进，冷启动或
  更大偏差步进并回写 SD2506，留痕 [SNTP] step 与 syslog OPER
- 首个校验通过的同步置 SYS_EVT_TIMESYNC_READY（就绪位，消费方按需 NoClear）
- [sntp] 配置段（enable/server1/server2/poll_min）属主 sntp_cfg.c，
  注册在 net_cfg_load()；UI 页 31「SNTP 设置」（密码叶）：启用开关 +
  主/备服务器 octet 编辑 + 周期(分) + 保存，复用 net_cfg_save_request 链
- 测试与门禁：test/host/sntp 新套件（报文/防伪/KoD/退避周期/阶跃域/配置
  注册表）、net_cfg 与 time_sync 套件补 fake、黄金参考 66→74 张零漂移、
  字符集 228 字、Keil 0 错 0 警（RO +5,848 B 含字体）、覆盖率基线回填
- 文档：墙钟维护说明 §16、网络配置说明 [sntp] 节、配置文件设计说明变更记录、
  产品需求 FR-TIME-3 与 8.3/9.1.2 台账、UI 补齐规划/UI补齐 31 叶口径、
  登记簿销项并登记「SNTP 校时真机判据」等台架（PC 时间源 192.168.31.161/.13）

scripts/test_coverage_baseline.json

@@ -208,13 +208,21 @@
       "lines_covered": 227,
       "lines_valid": 255
     },
+    "App/sync/time_sntp.c": {
+      "func_rate": 1.0,
+      "funcs_covered": 18,
+      "funcs_valid": 18,
+      "line_rate": 0.9153,
+      "lines_covered": 227,
+      "lines_valid": 248
+    },
     "App/sync/time_sync.c": {
       "func_rate": 1.0,
       "funcs_covered": 4,
       "funcs_valid": 4,
       "line_rate": 1.0,
-      "lines_covered": 100,
-      "lines_valid": 100
+      "lines_covered": 101,
+      "lines_valid": 101
     },
     "App/task/iec104_serv.c": {
       "func_rate": 0.75,
@@ -372,9 +380,17 @@
       "func_rate": 0.9375,
       "funcs_covered": 15,
       "funcs_valid": 16,
-      "line_rate": 0.9643,
-      "lines_covered": 108,
-      "lines_valid": 112
+      "line_rate": 0.9646,
+      "lines_covered": 109,
+      "lines_valid": 113
+    },
+    "Drivers/BSP/NETCFG/sntp_cfg.c": {
+      "func_rate": 0.75,
+      "funcs_covered": 3,
+      "funcs_valid": 4,
+      "line_rate": 0.8519,
+      "lines_covered": 23,
+      "lines_valid": 27
     },
     "Drivers/BSP/ScreenLink/sb_link.c": {
       "func_rate": 1.0,
@@ -560,9 +576,9 @@
     },
     "net_cfg": {
       "files": {
-        "Drivers/BSP/NETCFG/net_cfg.c": 112
+        "Drivers/BSP/NETCFG/net_cfg.c": 113
       },
-      "lines_total": 112,
+      "lines_total": 113,
       "ok": true
     },
     "net_events": {
@@ -685,6 +701,14 @@
       "lines_total": 114,
       "ok": true
     },
+    "sntp": {
+      "files": {
+        "App/sync/time_sntp.c": 248,
+        "Drivers/BSP/NETCFG/sntp_cfg.c": 27
+      },
+      "lines_total": 275,
+      "ok": true
+    },
     "soe": {
       "files": {
         "App/log/soe_rec.c": 87,
@@ -724,9 +748,9 @@
     },
     "time_sync": {
       "files": {
-        "App/sync/time_sync.c": 100
+        "App/sync/time_sync.c": 101
       },
-      "lines_total": 100,
+      "lines_total": 101,
       "ok": true
     },
     "ui_data": {
@@ -818,6 +842,7 @@
     "App/protocol/iec104/iec104_comm.c",
     "App/protocol/iec104/iec104_link.c",
     "App/protocol/iec104/iec104_pts.c",
+    "App/sync/time_sntp.c",
     "App/sync/time_sync.c",
     "App/task/iec104_serv.c",
     "App/util/boot_stats.c",
@@ -839,6 +864,7 @@
     "Drivers/BSP/NET/net_udp.c",
     "Drivers/BSP/NETCFG/ftp_cfg.c",
     "Drivers/BSP/NETCFG/net_cfg.c",
+    "Drivers/BSP/NETCFG/sntp_cfg.c",
     "Drivers/BSP/ScreenLink/sb_link.c",
     "Drivers/BSP/ScreenLink/sb_proto.c",
     "Drivers/BSP/UI/ui_data.c",
@@ -847,16 +873,16 @@
   ],
   "totals": {
     "files_generated_excluded": 5,
-    "files_tested": 52,
-    "files_untested": 173,
-    "func_rate": 0.9266,
-    "funcs_covered": 619,
-    "funcs_valid": 668,
-    "line_rate": 0.9012,
-    "lines_covered": 8372,
-    "lines_valid": 9290,
-    "modules_ok": 47,
-    "modules_total": 47
+    "files_tested": 54,
+    "files_untested": 177,
+    "func_rate": 0.9275,
+    "funcs_covered": 640,
+    "funcs_valid": 690,
+    "line_rate": 0.9014,
+    "lines_covered": 8624,
+    "lines_valid": 9567,
+    "modules_ok": 48,
+    "modules_total": 48
   },
   "untested_sources": [
     {
@@ -903,6 +929,10 @@
       "code_lines": 264,
       "path": "Drivers/BSP/SD2506/sd2506.c"
     },
+    {
+      "code_lines": 261,
+      "path": "Drivers/BSP/UI/ui_page_sntp.c"
+    },
     {
       "code_lines": 251,
       "path": "Drivers/BSP/UI/ui_page_wave_play.c"
@@ -1223,6 +1253,10 @@
       "code_lines": 25,
       "path": "Drivers/BSP/bsp_err.h"
     },
+    {
+      "code_lines": 24,
+      "path": "Drivers/BSP/NETCFG/sntp_cfg.h"
+    },
     {
       "code_lines": 24,
       "path": "Drivers/BSP/UI/ui_tokens.h"
@@ -1355,6 +1389,10 @@
       "code_lines": 12,
       "path": "App/app/app_main.h"
     },
+    {
+      "code_lines": 12,
+      "path": "App/sync/time_sntp.h"
+    },
     {
       "code_lines": 12,
       "path": "App/task/iec104_serv.h"
@@ -1427,6 +1465,10 @@
       "code_lines": 11,
       "path": "Drivers/BSP/UI/ui_page_reset.h"
     },
+    {
+      "code_lines": 11,
+      "path": "Drivers/BSP/UI/ui_page_sntp.h"
+    },
     {
       "code_lines": 11,
       "path": "Drivers/BSP/UI/ui_page_soe.h"