feat(ota): 实现GD32 OTA包间背压机制，优化传输节奏控制

移除固定的包间延迟配置 `OTA_PACKET_DELAY_MS`，采用基于SPI Link v2
CREDIT背压的动态节奏控制，在SpiMaster中通过remoteCredit==0时wait/拒发
实现流量控制，提高OTA传输稳定性。

refactor(stack): 统一任务栈大小配置并添加CPU统计功能

- 在app_config.h中集中定义所有任务栈大小常量，基于实测水位优化分配
- 新增monitor/cpu_stats.cpp组件，提供1Hz FreeRTOS CPU占用采样
- 更新main.cpp中的任务创建使用新的栈大小常量
- 调整health_monitor和ota_relay等任务的栈分配

perf(ota): 改进ESP32和GD32 OTA分区预擦策略

- ESP32 OTA从边写边擦改为整分区预擦模式，避免写入时的扇区擦除阻塞
- GD32 OTA在prepare阶段进行整分区预擦，提升后续写入吞吐量
- 优化OTA协调器状态管理，修复FINALIZING状态残留问题

fix(log): 改进UDP日志发送的堆内存管理

- 当堆内存不足时记录UDP发送跳过的告警信息到ring buffer
- 降低UDP发送跳过的内存阈值从10KB到6KB
- 降低日志排水最小内存阈值从8KB到4KB

docs(readme): 更新OTA配置参数文档说明

移除已废弃的OTA_PACKET_DELAY_MS参数，更新包间节奏控制说明为
基于CREDIT背压机制的描述。

main/Kconfig.projbuild

@@ -10,25 +10,17 @@ menu "DCU OTA Configuration"
             validated against actual wiring length/quality and GD32
             SPI slave timing.
 
-    config DCU_OTA_PACKET_INTERVAL_MS
-        int "Delay between GD32 OTA packets (ms)"
-        default 1
-        range 0 50
-        help
-            Idle delay inserted after each successfully acknowledged
-            firmware packet before sending the next one.
-            Lower value speeds up OTA, but if GD32 cannot finish
-            flash write / prepare next ACK in time, set it back.
-
     config DCU_OTA_POLL_BACKOFF_MS
         int "Polling backoff when GD32 is not ready (ms)"
         default 2
         range 0 50
         help
             Delay between SPI response polls when GD32 returns WAIT
-            or does not drive MISO yet. Lower value reduces per-packet
-            idle time and can significantly speed up OTA, but if GD32
-            cannot finish flash write before the next poll, increase it.
+            or does not drive MISO yet. Packet pacing uses SPI Link v2
+            CREDIT backpressure in SpiMaster (remoteCredit==0 → wait/
+            refuse TX), not a fixed inter-packet delay. Lower value
+            reduces poll idle time; increase if GD32 needs more time
+            between polls.
 
     config DCU_OTA_FINISH_TIMEOUT_MS
         int "Timeout for FINISH_OTA command (ms)"