fix(sha3): off-by-one in expand_message() loop counter #1053
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This change corrects the loop counter used when deriving b_i in expand_message_xmd for SHA3-256 to match RFC 9380 §5.3.1: b_1 is computed with I2OSP(1, 1) and subsequent blocks must use I2OSP(i, 1) for i in 2..=ell. The previous implementation iterated idx from 1 while computing b_2..b_ell but passed I2OSP(idx, 1), effectively labeling b_2 as 1 and shifting all subsequent block counters by one. Updating the counter to I2OSP(idx + 1, 1) ensures that b_2 is computed with 2, b_3 with 3, and so on, restoring spec compliance and interoperability with other implementations.