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/hub_test.go

@@ -32,7 +32,7 @@ func pipeClient(t *testing.T, h *Hub) (net.Conn, chan struct{}) {
 // once and returns ctx's error.
 func TestShutdownTimeout(t *testing.T) {
 	// No write timeout: the client's goodbye blocks until it is cut short.
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	_, done := pipeClient(t, h)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
@@ -49,7 +49,7 @@ func TestShutdownTimeout(t *testing.T) {
 
 // Shutdown with no clients returns at once, even with an expired ctx.
 func TestShutdownNoClients(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 	for range 20 {
@@ -64,7 +64,7 @@ func TestPingFailureDropsClient(t *testing.T) {
 	h := NewHub(config.JSCServerWebSocket{
 		PingInterval: 10 * time.Millisecond,
 		PongTimeout:  20 * time.Millisecond,
-	}, discardLog)
+	}, WithLogger(discardLog))
 	_, done := pipeClient(t, h)
 
 	select {
@@ -85,7 +85,7 @@ func TestMaxMessageSize(t *testing.T) {
 		{config.JSCServerWebSocket{MaxMessageSize: 100}, 100},
 	}
 	for _, tt := range tests {
-		h := NewHub(tt.conf, discardLog)
+		h := NewHub(tt.conf, WithLogger(discardLog))
 		if got := h.maxMessageSize(); got != tt.want {
 			t.Errorf("maxMessageSize(%d) = %d; want %d", tt.conf.MaxMessageSize, got, tt.want)
 		}