Error correction

Inner: Reed-Solomon over GF(256) per strand, with CRC-32 acceptance; native decoder with run-time AVX2 / AVX-512BW selection. Outer: Cauchy Reed-Solomon erasure code across strands in rows; the V6 product code adds column parity over stripes (opt-in). Opt-in recovery rounds: smart indel recovery, soft-decision decoding, retry band. Measured effect of each and why none is a default: docs/V6_DEFERRED.md.