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。

scripts/analyze_latency.py

@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+"""Summarize opt-in client marker reports using explicit, half-open row ranges."""
+import argparse
+import json
+import math
+from pathlib import Path
+import statistics
+
+
+def describe(values):
+    ordered = sorted(values)
+    if not ordered:
+        raise ValueError('No valid marker samples in the selected range')
+    return {
+        'count': len(ordered),
+        'mean': statistics.mean(ordered),
+        'p50': statistics.median(ordered),
+        'p95': ordered[int((len(ordered)-1)*.95)],
+        'p99': ordered[int((len(ordered)-1)*.99)],
+        'minimum': ordered[0],
+        'maximum': ordered[-1],
+    }
+
+
+def summarize(rows, selection):
+    start, end = selection['start_row'], selection['end_row']
+    if not 0 <= start < end <= len(rows):
+        raise ValueError('Range must select complete rows from the report')
+    samples = rows[start:end]
+    seen = set()
+    for sample in samples:
+        if not all(isinstance(v, (int, float)) and math.isfinite(v) for v in sample.values()):
+            raise ValueError('Sample contains a nonfinite value')
+        if not 0 <= sample['lowerMs'] <= sample['upperMs'] <= 5000:
+            raise ValueError('Invalid refresh bracket')
+        key = (sample['counter'], sample['sourceDrawMs'])
+        if key in seen:
+            raise ValueError('A source marker was counted more than once')
+        seen.add(key)
+    return {
+        **selection,
+        'samples': len(samples),
+        'sample_span_seconds': (samples[-1]['drawMs']-samples[0]['drawMs'])/1000,
+        'marker_event_to_paint_ms': describe([(s['lowerMs']+s['upperMs'])/2 for s in samples]),
+        'lower_bound_ms': describe([s['lowerMs'] for s in samples]),
+        'upper_bound_ms': describe([s['upperMs'] for s in samples]),
+        'refresh_bracket_width_ms': describe([s['upperMs']-s['lowerMs'] for s in samples]),
+        'decoded_to_draw_ms': describe([s['decodeToDrawMs'] for s in samples]),
+        'marker_analysis_ms': describe([s['analysisMs'] for s in samples]),
+    }
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--report', type=Path, required=True)
+    parser.add_argument('--range', type=Path, action='append', required=True, dest='ranges')
+    parser.add_argument('--out', type=Path, required=True)
+    args = parser.parse_args()
+    rows = [json.loads(line) for line in args.report.read_text().splitlines() if line.strip()]
+    trials = [summarize(rows, json.loads(path.read_text())) for path in args.ranges]
+    result = {
+        'method': '50 ms source marker events; first decodable echo on production VideoSurface; midpoint of adjacent-rAF brackets',
+        'scope': 'Source-screen marker to camera exposure/capture, Wi-Fi/UDP, native reassembly, local WebSocket, JPEG decode and canvas draw',
+        'limitations': [
+            'rAF brackets estimate display scheduling; panel scanout and pixel response are not measured',
+            'Mixed-exposure or unrecognized markers are excluded; first decodable echo can follow an earlier ambiguous image',
+            'Not every displayed frame is a sample; accepted samples describe this calibration scene and network',
+            'decodeToDrawMs starts after JPEG decoding, not at UDP or WebSocket arrival',
+        ],
+        'physical_panel_response_measured': False,
+        'trials': trials,
+    }
+    args.out.parent.mkdir(parents=True, exist_ok=True)
+    args.out.write_text(json.dumps(result, ensure_ascii=False, indent=2)+'\n', encoding='utf-8')
+    for trial in trials:
+        print(json.dumps({k: trial[k] for k in ('fb_count', 'samples', 'marker_event_to_paint_ms')}))
+
+
+if __name__ == '__main__':
+    main()