Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/websocket/tcp_test.go

@@ -31,7 +31,7 @@ func tcpHub(t *testing.T, mutate func(*config.JSCServerWebSocket)) (*Hub, string
 	if mutate != nil {
 		mutate(&conf)
 	}
-	h := NewHub(conf, discardLog)
+	h := NewHub(conf, WithLogger(discardLog))
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
@@ -243,7 +243,9 @@ func TestTCPPing(t *testing.T) {
 // A client that doesn't read is dropped as a slow consumer once its queue is
 // full. net.Pipe has no buffer, so writes to it block until the peer reads.
 func TestTCPSlowConsumer(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{WriteTimeout: 50 * time.Millisecond}, discardLog)
+	h := NewHub(
+		config.JSCServerWebSocket{WriteTimeout: 50 * time.Millisecond}, WithLogger(discardLog),
+	)
 	server, peer := net.Pipe()
 	t.Cleanup(func() { server.Close(); peer.Close() })
 	done := make(chan struct{})