C6: implementation, model error, and radiation loss

For adaptive decoding, retain the classical history in the complete step channels. A uniform error bound for every controlled branch bounds the entire controlled channel. A bound for a normalized postselected branch is not a complete-channel error bound.

For the two-slot benchmark decoder, seven unitary gates and two single-qubit destructive readouts give

when these are uniform complete-channel error bounds. This assumes compatible interfaces and no unmodelled correlated device memory. Radiation loss and encoding uncertainty are separate channel errors, not automatically gate noise.