feat: 链路控制、延迟校准与长测工具，补全文档

- 固件：新增 link_control / stream_control，覆盖运行中 Wi-Fi 断线恢复、取帧节拍、
  温控与质量/帧率减载；UDP 发送增加有界重试预算与帧期限，PSRAM JPEG 直发减少拷贝。
- 协议：重组器严格校验分片布局，统计未收齐、超时与完全缺失的帧，处理帧号回绕与设备重启。
- 桌面端：视频改 latest-value 通道并前置帧时间标记，拒绝积压和慢解码的过期画面；新增
  延迟标记页与 --latency-report 校准，完善录制、服务端与退出流程。
- 文档与脚本：新增硬件、开发、延迟、功耗、实时性、稳定性文档和改动记录，重写根 README；
  新增构建、性能、延迟与串口长测脚本，以及长测客户端脚本。
- 构建：固定 esp-1.97.0.0 工具链，分区表随构建复制，更新 sdkconfig 与忽略规则。

components/esp32-camera/driver/cam_hal.c

@@ -733,13 +733,14 @@ camera_fb_t *cam_take(TickType_t timeout)
     for (;;)
     {
         TickType_t elapsed = xTaskGetTickCount() - start; /* TickType_t is unsigned so rollover is safe */
-        if (elapsed >= timeout) {
+        if (timeout && elapsed >= timeout) {
             ESP_LOGW(TAG, "Failed to get frame: timeout");
             return NULL;
         }
-        TickType_t remaining = timeout - elapsed;
+        TickType_t remaining = timeout ? timeout - elapsed : 0;
 
         if (xQueueReceive(cam_obj->frame_buffer_queue, (void *)&dma_buffer, remaining) == pdFALSE) {
+            if (!timeout) return NULL;
             continue;
         }
 
@@ -750,6 +751,7 @@ camera_fb_t *cam_take(TickType_t timeout)
             if (dma_reset_counter < MAX_GDMA_RESETS) {
                 ll_cam_dma_reset(cam_obj);
                 dma_reset_counter++;
+                if (!timeout) return NULL;
                 continue; /* retry with queue timeout */
             }
             if (dma_reset_counter == MAX_GDMA_RESETS) {
@@ -762,6 +764,7 @@ camera_fb_t *cam_take(TickType_t timeout)
             CAM_WARN_THROTTLE(warn_null_cnt,
                               "Unexpected NULL frame on " CONFIG_IDF_TARGET);
 #endif
+            if (!timeout) return NULL;
             vTaskDelay(1); /* immediate yield once resets are done */
             continue;             /* go to top of loop */
         }
@@ -802,6 +805,7 @@ skip_eoi_check:
             CAM_WARN_THROTTLE(warn_eoi_miss_cnt,
                               "NO-EOI - JPEG end marker missing");
             cam_give(dma_buffer);
+            if (!timeout) return NULL;
             continue; /* wait for another frame */
         } else if (cam_obj->psram_mode &&
                    cam_obj->in_bytes_per_pixel != cam_obj->fb_bytes_per_pixel) {
@@ -818,6 +822,24 @@ skip_eoi_check:
     }
 }
 
+camera_fb_t *cam_take_latest(TickType_t timeout, uint32_t *skipped)
+{
+    if (skipped) *skipped = 0;
+    camera_fb_t *latest = cam_take(timeout);
+    if (!latest) return NULL;
+    /* A bounded drain, not availability-check + blocking get: the producer can
+     * replace queued buffers on the other core. Retain the valid frame if a
+     * newer candidate is invalid or a GDMA-reset sentinel is encountered. */
+    for (size_t i = 1; i < cam_obj->frame_cnt; i++) {
+        camera_fb_t *next = cam_take(0);
+        if (!next) break;
+        cam_give(latest);
+        latest = next;
+        if (skipped) (*skipped)++;
+    }
+    return latest;
+}
+
 void cam_give(camera_fb_t *dma_buffer)
 {
     for (int x = 0; x < cam_obj->frame_cnt; x++) {