Keep neoirc-cli sessions on localhost and test the relay goroutine stop

The auth cookie is always Secure, and Go 1.24's cookie jar sends a
Secure cookie only over HTTPS, so neoirc-cli lost its session against a
plain-HTTP server on localhost. Its jar now treats a server on localhost
as HTTPS, as curl does. The README's Transport Security section and
Python example say what a client needs.

A test checks that closing a registered IRC client's connection stops
its relay goroutine. The README Entrypoints section names a make target
only for the scripts that have one, and the Dockerfile says why the
test phase runs with -p 4.

Model: opus-5-5

internal/ircserver/server_test.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"net"
 	"os"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -608,6 +609,56 @@ func TestNamesNonExistentChannel(t *testing.T) {
 	)
 }
 
+// TestRelayStopsWhenConnectionCloses checks that closing a
+// registered client's connection stops the goroutine that
+// relays its messages.
+//
+//nolint:paralleltest // counts every goroutine in the process
+func TestRelayStopsWhenConnectionCloses(t *testing.T) {
+	env := newTestEnv(t)
+	client := env.dial(t)
+
+	client.register("relaystop")
+	waitForRelayGoroutines(t, 1)
+
+	err := client.conn.Close()
+	if err != nil {
+		t.Fatalf("close: %v", err)
+	}
+
+	waitForRelayGoroutines(t, 0)
+}
+
+const (
+	stackDumpSize  = 1 << 20
+	relayCheckStep = 10 * time.Millisecond
+)
+
+// waitForRelayGoroutines waits until exactly want goroutines
+// are running the relay loop, and fails the test if that does
+// not happen within testTimeout.
+func waitForRelayGoroutines(t *testing.T, want int) {
+	t.Helper()
+
+	deadline := time.Now().Add(testTimeout)
+	stacks := make([]byte, stackDumpSize)
+
+	for {
+		n := runtime.Stack(stacks, true)
+		got := strings.Count(string(stacks[:n]), "(*Conn).relayMessages(")
+
+		if got == want {
+			return
+		}
+
+		if time.Now().After(deadline) {
+			t.Fatalf("relay goroutines: got %d, want %d", got, want)
+		}
+
+		time.Sleep(relayCheckStep)
+	}
+}
+
 func BenchmarkParseMessage(b *testing.B) {
 	line := ":nick!user@host PRIVMSG #channel :Hello, world!"