Interface RunnerCallback

The zero-argument shape @gateline/server's own RunnerCallback expects (runner-api.ts) — mirrored structurally here rather than imported, so this package carries no edge to @gateline/server (the same no-edge convention runner-api.ts documents in the other direction, toward this package). Structurally assignable to @gateline/server's RunnerCallback — a caller assembling both (main.ts) passes a value of this shape straight into ServeOptions.runnerCallback.