nx_png_dynamic_huffman_test.nx

nx_png_dynamic_huffman_test.nx -- regression smoke proving nx_png_decode can consume a real-world dynamic-Huffman zlib stream (the format every gen-img PNG uses). SURFACED 2026-05-18 via empirical run: nx_image_grade_cli reports IGV2_ERR_DECODER=3 on any real PNG. The companion smoke nx_deflate_huffman_test.nx proved nx_deflate_inflate handles the SAME 61-byte raw DEFLATE bytes when called directly. So the bug is in nx_png_decode (or in how it hands IDAT bytes to nx_zlib_inflate). Fixture: 124-byte PNG built by python zlib.compress(raw, 6) where raw = 50 scanlines of "0x00 [200,170,150]*50". IDAT inner zlib stream = 67 bytes (0x78 0x9c header + 61 raw DEFLATE + 4 adler). Expected decode: width=50, height=50, pixels_size = 50*50*3 = 7500.