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。

desktop/scripts/test-live-freshness.mjs

@@ -0,0 +1,41 @@
+// Exercise the production StreamClient against native hardware video, while
+// deliberately stalling only this auxiliary local consumer. Adds no UDP peer.
+// Bitmap decoding is stubbed here; actual JPEG decoding is checked separately.
+import {readFileSync,writeFileSync} from 'node:fs';
+import assert from 'node:assert/strict';
+import ts from 'typescript';
+
+const [logPath,outPath]=process.argv.slice(2);
+if(!logPath||!outPath)throw new Error('Use <private native debug log> <result JSON>');
+const log=readFileSync(logPath,'utf8');
+const url=[...log.matchAll(/ws:\/\/127\.0\.0\.1:\d+\/\?token=[A-Za-z0-9_-]+/g)].at(-1)?.[0];
+if(!url)throw new Error('No local video URL in the private debug log');
+const source=readFileSync(new URL('../src/lib/stream.ts',import.meta.url),'utf8');
+const compiled=ts.transpileModule(source,{compilerOptions:{module:ts.ModuleKind.CommonJS,target:ts.ScriptTarget.ES2022}}).outputText;
+const exports={};new Function('require','exports',compiled)(()=>({FRAME_HEADER_LEN:16}),exports);
+globalThis.window={setTimeout,clearTimeout};
+globalThis.createImageBitmap=async()=>({close(){}});
+const rx=new exports.StreamClient({onMessage(){},onConnection(){}});
+const delay=ms=>new Promise(resolve=>setTimeout(resolve,ms));
+try {
+  rx.connect(url);
+  const deadline=performance.now()+10_000;
+  while(rx.counters.frames<10&&performance.now()<deadline)await delay(30);
+  assert.ok(rx.counters.frames>=10,'Real native JPEG video did not arrive');
+  const camera=[...rx.frames.keys()][0];
+  const before=rx.latest(camera)?.frameId;
+  const staleBefore=rx.counters.staleFrames;
+  const blockedAt=performance.now();
+  while(performance.now()-blockedAt<350) { /* synthetic consumer stall */ }
+  await delay(700);
+  const latest=rx.latest(camera);
+  assert.ok(rx.counters.staleFrames>staleBefore,'Stale native-timestamped frames were not rejected');
+  assert.ok(latest&&latest.frameId!==before,'The consumer did not recover to new video');
+  assert.ok(performance.now()-latest.receivedAt<150,'Recovered image is still stale locally');
+  const result={passed:true,stall_ms:350,stale_frames_rejected:rx.counters.staleFrames-staleBefore,
+    frame_before:before,frame_after:latest.frameId,frames_received:rx.counters.frames,
+    additional_udp_subscribers:0,jpeg_decoder_stubbed:true,
+    scope:'Production StreamClient over real native WebSocket; tests queued-frame age and recovery, not physical display latency'};
+  writeFileSync(outPath,JSON.stringify(result,null,2)+'\n');
+  console.log(JSON.stringify(result));
+} finally {rx.close();}