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。

.env.example

@@ -20,6 +20,54 @@ WIFI_PASS=your-password
 #CAM_RES=vga
 # JPEG quality 0-63, lower is better quality
 #CAM_QUALITY=12
+# Optional adaptive compression ceiling, 0 = fixed (default), else QUALITY..63.
+# Shed load on sender congestion, restore detail after 15 stable seconds.
+#CAM_QUALITY_MAX=0
+# Stream cap, 0 = uncapped (default), 1-60 = maximum fps.
+# Paces frame retrieval and UDP delivery; sensor clocks remain unchanged.
+#CAM_MAX_FPS=0
+# OV5640 PLL profile: default (driver timing) | vga30 (VGA ~30.09 fps).
+# vga30 preserves HTS/VTS: XCLK 20, VCO 610, SYSCLK 61, PCLK 15.25 MHz.
+# Retune exposure-line limits when switching profiles; keep HTS at default.
+#CAM_CLOCK_PROFILE=default
+# Capture frame buffers in PSRAM: 3 (default) or 2 (lower capture queue depth).
+# Prefer 3 for burst tolerance; 2 showed worse long-run peak frame loss here.
+#CAM_FB_COUNT=3
+# Sensor grayscale effect while retaining hardware JPEG, 0 = colour (default).
+#CAM_GRAYSCALE=0
+# OV5640 AE target bias: 0 (default), -1 (~-1.1 EV), -2 (~-1.9 EV).
+# This target alone does not guarantee a shorter shutter.
+#CAM_EXPOSURE_EV=0
+# Optional OV5640 driver AE level -5..5; requires CAM_EXPOSURE_EV=0.
+# Unset uses the EV setting. -1 lowers the target 55 -> 45 (~-0.29 EV),
+# a finer brightness adjustment than CAM_EXPOSURE_EV=-1. AEC/AGC stay on.
+#CAM_AE_LEVEL=-1
+# OV5640 fixed shutter in sensor lines, 0 = automatic (default), AGC stays on.
+# 220 lines is roughly 10 ms with default VGA HTS 2060; verify timing/logs.
+#CAM_SHUTTER_LINES=0
+# Optional automatic exposure cap in sensor lines, 0 = driver default.
+# AEC/AGC stay automatic, night frame extension is disabled. Mutually exclusive
+# with a nonzero fixed shutter. Default VGA: 440 lines is about 20 ms.
+# With vga30 use 592 lines for about 20 ms, including two 50 Hz AEC bands.
+#CAM_EXPOSURE_MAX_LINES=0
+# OV5640 automatic gain ceiling, 0 = driver default, else 1..63 times.
+# More gain can brighten short exposures but also amplifies noise.
+#CAM_GAIN_CEILING=0
+# OV5640 horizontal total, 0 = driver default. Even values only; hardware test
+# required. Changing HTS also changes the time represented by one shutter line.
+# HTS 1500 / 1800 caused green/magenta flicker on this module; keep 0.
+#CAM_HTS=0
+# Minimum gap between successful video datagrams, 0 = burst (default).
+# Allowed 0..5000 us in FreeRTOS tick increments (1000 us in this build).
+#UDP_PACKET_GAP_US=0
+# Total yielding-retry allowance per JPEG, shared by all fragments/peers.
+# 0 = fail on backpressure; default 8000 us, 0..25000 in tick increments.
+# Frame deadlines remain 25 ms sending / 100 ms since capture start.
+#UDP_RETRY_BUDGET_US=8000
+# Internal ESP chip temperature load limit, Celsius; 0 disables, default 75.
+# At the limit halve fps (rounded up); restore after cooling by 5 C.
+# This is a proactive load threshold, not a specified silicon safety limit.
+#CAM_THERMAL_LIMIT_C=75
 # Set to 0 if your board inverts VSYNC in hardware.
 #CAM_VSYNC_INVERT=1
 
@@ -28,3 +76,13 @@ WIFI_PASS=your-password
 # the AP (a hidden SSID, for instance) - otherwise the scan reports the real
 # mode and this only makes it fail faster.
 #WIFI_AUTH=wpa2
+# Optional maximum Wi-Fi TX power, integer dBm (2..20). Unset keeps SDK default.
+# Test on your own board/network; higher power does not always improve video.
+# Current delivery requests 15 dBm; this ESP32-S3 reports a 15.00 dBm cap.
+# Runtime load control does not lower it; reconnects reapply the same cap.
+# WIFI_TX_POWER_DBM=15
+
+# Diagnostic builds only: intentionally disconnect once, after N seconds of
+# the streaming loop, hold offline 5 seconds, then exercise runtime recovery.
+# Keep 0 (default) for normal firmware; does not change router/TUN settings.
+#WIFI_TEST_DROP_AFTER_S=0