nx_u256_mul_test.nx

nx_u256_mul_test.nx -- KAT for the 256x256 -> 512 wide-multiply. Verifies: - 0 * 0 = 0 - 1 * 1 = 1 - a * 1 = a (identity) - 1 * a = a (commutative-with-identity) - 2^32 * 2^32 = 2^64 (cross-limb single bit) - (2^256 - 1)^2 produces the well-known wide value - small * small produces known products (3 * 5 = 15, etc.) - large limb-spanning case (0xFFFFFFFF * 0xFFFFFFFF) - wide_cmp / wide_fits_in_256 / wide_copy_low helpers expect_exit: 0 license_tier: ORIGINAL