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.go

@@ -71,10 +71,10 @@ func (h *Hub) ServeTCP(ctx context.Context, newConn net.Conn) {
 	}
 	defer h.unregister(client)
 
-	h.log.Info("client connected", "client", client.id, "ip", ip, "transport", "tcp")
+	h.logger().Info("client connected", "client", client.id, "ip", ip, "transport", "tcp")
 	client.run(ctx)
 	conn.CloseNow() //nolint:errcheck,gosec // finished with; it may already be closed
-	h.log.Info("client disconnected", "client", client.id, "ip", ip, "transport", "tcp")
+	h.logger().Info("client disconnected", "client", client.id, "ip", ip, "transport", "tcp")
 }
 
 // ---------- tcpConn ----------