Support local (inline) IIIF manifests without fetching them

Support local (inline) IIIF manifests without fetching them: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). Right now every manifest Mirador knows about is identified by a URL and is loaded by fetching that URL over the network. We're adding the ability to hand Mirador a…