Callable contract interfaces and @typescript-eslint/prefer-function-type

type-alias-invariants requires callable contracts to use interfaces. @typescript-eslint/prefer-function-type can object when such an interface contains only a call signature.

Use an optional unique-symbol brand member to make the interface a contract with an explicit nominal marker:

export interface LoaderInterface {
  (iri: string): Promise<string | null>;
  readonly 'loaderBrand'?: unique symbol;
}

The optional member does not require a function value to carry the property, while the interface remains a callable contract. type-alias-invariants accepts the interface form, and the additional member means prefer-function-type does not see an interface containing only a call signature.

This is a consumer-side declaration pattern, not a rule configuration requirement.