fix(detect): never bind USB gadget iface as proxy egress

设备把自己当 USB 网卡挂给上游主机时会出现 usb0（驱动 configfs-gadget.g1，
IP 192.168.68.1）。该链路只通向 PC，被当成出口后所有出站都 context
deadline exceeded，客户端看到 socks connect failed rep=0x01。

误选原因：score_iface_as_cellular 给 usb0 与真出口 wwan0 都打 175 分。
usb0 靠名称命中 patterns 里的 usb（+80）与非默认路由（+40）；wwan0 的
bam-dmux 驱动不在白名单里，白丢 +100。detect_cellular_iface 用 -gt 比较，
list_ifaces 按名排序让 usb0 先入选，平分下无法被顶替。

改动：
- lib.sh 新增 iface_is_usb_gadget()，按驱动名与 gadget 总线路径识别，
  并并入 is_virtual_or_skip_iface（usb0 评分 175 -> 0）
- 驱动白名单补 bam-dmux/bam_dmux/qcom-ipa/ipa_wan；名称权重 80 -> 40，
  确保驱动证据始终压过名称猜测
- 新增 detect_cellular_iface_via_mm()，把 ModemManager bearer 的
  interface: 作为第 0 步权威来源
- detect_cellular_iface / resolve_cellular 全链路拒绝 gadget 网卡，
  取不到真出口时按 REQUIRE_CELLULAR_IFACE 直接失败而不是绑错
- generate/install/upgrade/watch/detect/verify 各入口独立设闸，
  verify.sh 发现出口是 gadget 时直接 exit 4 并给出修复命令
- 默认 CELLULAR_IFACE_PATTERNS 去掉 usb/enx；upgrade.sh 与根 install.sh
  就地迁移已有 settings.conf，旧机器升级即修复
- 新增 tests/detect-gadget.sh：18 条离线断言，无需真机

scripts/watch-cellular.sh

@@ -255,6 +255,16 @@ need_refresh() {
     return 0
   fi
 
+  if iface_is_usb_gadget "$cell"; then
+    logw "CELLULAR_IFACE=$cell is a USB gadget link (host-facing), never a data egress → rebind"
+    return 0
+  fi
+
+  if [[ -n "$cfg_iface" ]] && iface_is_usb_gadget "$cfg_iface"; then
+    logw "config bind_interface=$cfg_iface is a USB gadget link → rebind"
+    return 0
+  fi
+
   live_src="$(detect_source_ip "$cell" || true)"
   if [[ -z "$live_src" ]]; then
     # 网卡在但无 IPv4：可能刚重拨；不立刻 die，等下一轮或 MM 恢复路径
@@ -290,10 +300,13 @@ do_refresh() {
   local src=""
   local reason="${1:-manual}"
 
-  if [[ -z "$cell" ]] || ! iface_exists "$cell"; then
+  if [[ -z "$cell" ]] || ! iface_exists "$cell" || iface_is_usb_gadget "$cell"; then
+    if [[ -n "$cell" ]] && iface_is_usb_gadget "$cell"; then
+      logw "refuse USB gadget iface $cell as egress; re-detecting"
+    fi
     cell="$(detect_cellular_iface "" || true)"
-    if [[ -z "$cell" ]]; then
-      logw "detect failed; skip reason=$reason"
+    if [[ -z "$cell" ]] || iface_is_usb_gadget "$cell"; then
+      logw "detect failed or gadget-only; skip reason=$reason"
       return 1
     fi
   fi