nx_self_similarity_test.nx

nx_self_similarity_test.nx -- smoke for box-counting fractal dim. Per the substrate's bench-press cardinal: prove the primitive on hand-constructed images with known fractal properties, before any caller relies on it. Test cases: 1. Uniform image (no edges) -> refused with fidelity = 0 2. Checkerboard (every cell is an edge) -> high D ~ 2.0 3. Single horizontal line -> D ~ 1.0 (low) 4. Diagonal stripes at moderate spacing -> D in the Spehar band 5. Spehar peak-band predicate detects band 1.3-1.5 correctly 6. Report fields all in [0, 1024] range