nx_dequant_iter_test.nx

nx_dequant_iter_test.nx -- smoke for nx_dequant_iter.nx. Builds a synthetic Q4_K super-block in memory with known scales, mins, and nibble values. Tests: A) Verdict gate B) Iter init reads d/dmin/scales/mins correctly C) nx_q4k_iter_value at known indices matches expected formula D) nx_q4k_iter_subblock_dequant matches nx_gguf_dequant_q4_k bit-exactly for all 256 values E) Out-of-range index returns 0 (no crash)