watcher: an IPv6 address is written in brackets before its port (closes #270)

Port and certificate keys were joined as address:port, so an IPv6
address ran into its port: 2a01:4f8:2200:fd11::1:80 is also an address
on its own. Keys are now joined with net.JoinHostPort and split with
net.SplitHostPort, giving [2a01:4f8:2200:fd11::1]:80 in the state file,
/api/v1/status, the dashboard and Port Change notifications; an IPv4
address is unchanged. A certificate key still splits at its last colon,
since a hostname has none. Tests that built IPv6 keys by hand now use
the bracketed form, and a test checks that an IPv6 address's
certificate entry is removed when its port 443 closes.

Model: opus-5-5

internal/watcher/removed_test.go

@@ -2,6 +2,7 @@ package watcher_test
 
 import (
 	"maps"
+	"net"
 	"slices"
 	"strings"
 	"testing"
@@ -173,7 +174,7 @@ func TestCertificateStateForAnAddressGone(t *testing.T) {
 					"A": {ip1}, "AAAA": {ip6},
 				}),
 			}),
-			[]string{ip1 + ":443:" + host, ip6 + ":443:" + host},
+			[]string{ip1 + ":443:" + host, "[" + ip6 + "]:443:" + host},
 		},
 		{
 			"no nameserver answered",
@@ -183,7 +184,7 @@ func TestCertificateStateForAnAddressGone(t *testing.T) {
 			[]string{
 				ip1 + ":443:" + host,
 				ip2 + ":443:" + host,
-				ip6 + ":443:" + host,
+				"[" + ip6 + "]:443:" + host,
 			},
 		},
 	}
@@ -206,7 +207,8 @@ func TestCertificateStateForAnAddressGone(t *testing.T) {
 
 			for _, ip := range []string{ip1, ip2, ip6} {
 				deps.state.SetCertificateState(
-					ip+":443:"+host, &state.CertificateState{Status: "ok"},
+					net.JoinHostPort(ip, "443")+":"+host,
+					&state.CertificateState{Status: "ok"},
 				)
 			}