Free internal transfer (no chain touched)

Moves value between two enrolled accounts as a book entry: no gas, no bridge, no fee, and NO transaction hash because no transaction occurs. Both parties must be enrolled — we will not hold a balance for a recipient who never consented. Sign robyn-netting-transfer:v1:from=…:to=…:amount=…:nonce=… with the sender key. Nonces are monotonic, so a captured signature cannot be replayed.