feat(vd960Loop): 看门狗升级双保险喂狗模式 + 超时对齐 V4B（1.64s）

对照 DLD154V4B V2.10（同款 AT32F421）:
- 原直喂模式缺 ISR 活性检测: TMR15 ISR 死但主循环活 -> 看门狗永不复位(假活)
- 升级: TMR15 ISR 5ms 递增 g_wdg_counter + 主循环 poll_wdg() 500ms 才 reload
- 超时 3s(div64/rld1875) -> 1.64s(div16枚举/rld4095)
- wdt_feed 定义移至 TaskLoop.c 改名 poll_wdg, 与 V4B 同构
- 修正 main.c 注释矛盾(4687 vs 1875); 沿用 WDT_CLK_DIV_16 枚举写法避免
  V4B 同源传值 bug
- 隔离测试 3 项断言全过(分频16/超时1638ms>500ms/10s喂20次/ISR死不喂)

DLD960LoopBootloader/libraries/drivers/inc/at32f421_def.h

@@ -0,0 +1,69 @@
+/**
+  **************************************************************************
+  * @file     at32f421_def.h
+  * @brief    at32f421 macros header file
+  **************************************************************************
+  *                       Copyright notice & Disclaimer
+  *
+  * The software Board Support Package (BSP) that is made available to
+  * download from Artery official website is the copyrighted work of Artery.
+  * Artery authorizes customers to use, copy, and distribute the BSP
+  * software and its related documentation for the purpose of design and
+  * development in conjunction with Artery microcontrollers. Use of the
+  * software is governed by this copyright notice and the following disclaimer.
+  *
+  * THIS SOFTWARE IS PROVIDED ON "AS IS" BASIS WITHOUT WARRANTIES,
+  * GUARANTEES OR REPRESENTATIONS OF ANY KIND. ARTERY EXPRESSLY DISCLAIMS,
+  * TO THE FULLEST EXTENT PERMITTED BY LAW, ALL EXPRESS, IMPLIED OR
+  * STATUTORY OR OTHER WARRANTIES, GUARANTEES OR REPRESENTATIONS,
+  * INCLUDING BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY,
+  * FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT.
+  *
+  **************************************************************************
+  */
+
+/* Define to prevent recursive inclusion -------------------------------------*/
+#ifndef __AT32F421_DEF_H
+#define __AT32F421_DEF_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* gnu compiler */
+#if defined (__GNUC__)
+  #ifndef ALIGNED_HEAD
+    #define ALIGNED_HEAD
+  #endif
+  #ifndef ALIGNED_TAIL
+    #define ALIGNED_TAIL                 __attribute__ ((aligned (4)))
+  #endif
+#endif
+
+/* arm compiler */
+#if defined (__CC_ARM)
+  #ifndef ALIGNED_HEAD
+    #define ALIGNED_HEAD                 __align(4)
+  #endif
+  #ifndef ALIGNED_TAIL
+    #define ALIGNED_TAIL
+  #endif
+#endif
+
+/* iar compiler */
+#if defined (__ICCARM__)
+  #ifndef ALIGNED_HEAD
+    #define ALIGNED_HEAD
+  #endif
+  #ifndef ALIGNED_TAIL
+    #define ALIGNED_TAIL
+  #endif
+#endif
+
+#define UNUSED(x)                        (void)x /* to avoid gcc/g++ warnings */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif