AssetLoader typedef
Signature for a function that loads an SVG fragment by its asset key.
Asset keys are bare relative paths within the assets directory,
e.g. eyes/close.svgf or shared_defs.svgf.
The loader is responsible for resolving the key to a concrete location (filesystem path, Flutter asset bundle key, etc.) and returning the string content.
Return an empty string for empty keys (used by "blank" variants that have no SVG fragment).
Implementation
typedef AssetLoader = Future<String> Function(String assetKey);