Add a Limits type to LoadingImage that controls which transmission mediums (file, temporary_file, shared_memory) are allowed when loading images. This defaults to "direct" (most restrictive) on ImageStorage and is set to "all" by Termio, allowing apprt embedders like libghostty to restrict medium types for resource or security reasons. The limits are stored on ImageStorage, plumbed through Screen.Options for screen initialization and inheritance, and enforced in graphics_exec during both query and transmit. Two new Terminal methods (setKittyGraphicsSizeLimit, setKittyGraphicsLoadingLimits) centralize updating all screens, replacing the manual iteration previously done in Termio.