Add the five-part learn/ folder (overview, concepts, architecture,
implementation, challenges) grounded in docs/research, with the
QR-from-ISO/IEC-18004 Reed-Solomon injection as the showpiece and crypha's
real capacities cited from the binary. Update the root README row from a
Python synopsis to the built Go project with Source Code and Docs links,
matching the nadezhda row.
CarterPerez-dev