fix(ftp): 控制通道超时改字节间隔语义，保活 NOOP 可续命会话

- lftpd_inet_read_line 预算原为「整行总时长」：t0 只在入口置一次，
  保活型客户端的零散短行会被 30s 总预算误掐，会话周期被关闭
- 改为任何字节到达即重置 t0（字节间隔超时）；空闲回收语义不变，
  零字节静默满预算仍判超时释放
- 台架 2026-11-13：FileZilla 下载失败三方互证闭环——MobaXterm
  单连接成功、本地探针服务器捕获 FileZilla 第二条控制连接行为、
  ftplib 主机侧下载/上传全过
- 编译 0 错误 0 警告（keil_build）

scripts/size_attestation.json

@@ -1,15 +1,15 @@
 {
-  "built_at": "2026-11-13T11:25:00+08:00",
-  "fingerprint": "39f4f69a673f97c33cc59b301e723ec8f1e03686f4556ac6abb1a5a6cd438796",
+  "built_at": "2026-11-13T16:20:25+08:00",
+  "fingerprint": "3c4a1381e6e0306968e2a4ce03a2d25fd72f65a7262234e7e15703599301a54d",
   "fingerprint_files": 1479,
   "fingerprint_source": "git-index",
   "gated_regions": [
     "RW_IRAM1",
     "RW_CCM"
   ],
-  "git_head": "7c505b2457d977adf7750abbb841b0453fd6bfb9",
+  "git_head": "40a2816a9971928d1ca6fa218921b86dae79bd63",
   "map": "MDK-ARM/STM32F407-Demo/STM32F407-Demo.map",
-  "map_mtime": 1794540294,
+  "map_mtime": 1794550493,
   "pct": {
     "ER_IROM1": 27.46,
     "RW_CCM": 87.89,
@@ -19,7 +19,7 @@
   "regions": {
     "ER_IROM1": {
       "max": 1048576,
-      "size": 287964
+      "size": 287968
     },
     "RW_CCM": {
       "max": 65280,
@@ -37,7 +37,7 @@
   "schema": 2,
   "scope_note": "\u540c\u7801\u51ed\u8bc1\uff1a\u6307\u7eb9\u8986\u76d6\u56fa\u4ef6\u4f53\u79ef\u76f8\u5173\u6e90\u7801\uff0c\u53e3\u5f84\u89c1\u672c\u811a\u672c docstring",
   "sizes": {
-    "code": 256200,
+    "code": 256204,
     "ro_data": 31764,
     "rw_data": 2080,
     "zi_data": 152368