feat(agent): 配对窗口可显式绑定地址，默认仍是内网可达

配对窗口是 agent 唯一持有的监听端口，而它授予的是设备信任：谁能带节点密码打到
/provision，谁就拿到一把 agent 会听命的密钥。此前它固定绑所有接口，且没有任何办法
收窄——一台本身能从公网路由到的节点（公网 VPS 上的 SimAdmin、放在非可信网络里的
设备）在窗口开启期间就把这个面暴露给了整个互联网。

现在加 --pair-bind，只接受 IP 字面量。不接受主机名是刻意的：绑定时解析名字意味着
监听地址取决于 DNS 当时说什么，那不是运维做过的决定。

默认值仍是绑所有接口，这个我改过一次又改回来了，原因值得记下来：我先把默认设成
127.0.0.1 并写了理由，然后发现配对按设计走内网、手机要直接打到节点，回环监听从
另一台机器根本连不上——那不是一个加固措施，而是一个永远不工作的流程，用户对这个
功能的第一次体验会是"连接被拒"且没有解释。所以默认保持内网可达，把"节点能从公网
路由到时请收窄"写进部署文档，并在 --pair-bind 上给出显式入口。

测试断言默认值同时满足两件事：是 IP 字面量（不是名字），且不是回环（手机要能连）。

Go 全包、Node 120 项、Swift 107 项通过。

internal/pair/pair_test.go

@@ -6,6 +6,7 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"encoding/json"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -280,3 +281,25 @@ func TestServeDeadlineClosesTheWindow(t *testing.T) {
 		t.Fatalf("status after the deadline: %d", response.StatusCode)
 	}
 }
+
+// The window's default bind has to satisfy two things at once, and they pull in opposite directions.
+//
+// It must be reachable from the phone: pairing happens over the LAN by design, so a loopback default
+// would be a flow that never works. And it must be a literal rather than a name: a name is resolved at
+// bind time, so the listening address would depend on what DNS said, which is not a decision the
+// operator made.
+func TestTheDefaultBindIsALiteralAndIsReachableFromTheLAN(t *testing.T) {
+	if net.ParseIP(DefaultAdminHost) == nil {
+		t.Fatalf("%q is not an IP literal; a name would be resolved at bind time", DefaultAdminHost)
+	}
+	ip := net.ParseIP(DefaultAdminHost)
+	if ip.IsLoopback() {
+		t.Fatal("a loopback default cannot be reached from the phone, and pairing is a LAN flow")
+	}
+	// Any-address is the default because the documented flow requires it; the operator narrows it with
+	// --pair-bind when the node is routable from somewhere it should not be.
+	if !ip.IsUnspecified() {
+		t.Fatalf("the default bind changed to %q; the documented flow and the deployment notes assume %q",
+			DefaultAdminHost, "0.0.0.0")
+	}
+}