Optionalextensions: Record<string, string>URL of a model file for the given (modelId, format) pair.
Studio's loadModel calls this when the caller didn't supply an
explicit src.
URL of a sidecar file beside the model's directory — files that
aren't themselves model formats but belong logically to the
model (currently just coordSys.json; pattern extends to any
future per-model metadata).
Built-in locator for the demo convention
{modelsDir}/{modelId}/{format}/model.{ext}.Owns the format → extension map so that loader modules carry no filename or path information. Callers that need a different layout inject their own ModelLocator via StudioConfig.