nx_jpeg_dequant.nx

nx_jpeg_dequant.nx -- dequantization + un-zigzag composer. Per ITU-T Rec. T.81 sec A.3.4 / F.2.1.4. JPEG stores quantized DCT coefficients in zig-zag scan order so the entropy-coded RLE clusters the typically-many trailing zeros. To recover natural-order coefficients for the IDCT: 1. Dequantize: natural-magnitude_zz[k] = q_zz[k] * quant_zz[k] (element-wise multiply in zig-zag order; legal because both arrays are in the same order) 2. Un-zigzag: natural[un_zigzag[k]] = natural-magnitude_zz[k] Composed in a single pass for efficiency: walk k = 0..63 once, computing the natural position from the un_zigzag table and writing the dequantized value directly to natural[un_zigzag[k]]. nx_safety_envelope: intended_use: "Dequant + un-zigzag for JPEG decode pipeline." sil_target: SIL1 evidence: [t81_section_a3_4_canonical_basis, composes_zigzag_primitive, composes_dqt_primitive] hazard_register: [bug-tape-dequant-zero-divisor-not-applicable, bug-tape-zigzag-permutation-table-corruption] residual_risk: "Caller responsible for ensuring quant table has 64 entries and is the table-id specified by the SOF0 component descriptor." verdict: NOT_YET_EVALUATED