Visualization CLI

gat viz plot is a placeholder that runs the gat-viz helper and prints a summary string. It exists so visualization primitives stay wired into the CLI for future exporters.

gat viz plot test_data/matpower/ieee14.arrow --output out/example.png

The current backend returns a static string (gat_viz::visualize_data()), but the command demonstrates how the visualization helpers can hook into the CLI and default logging. Use it as a pattern when adding real SVG/PNG/Parquet exporters later.