ZK transfers as first-class citizens in vanta-explorer

The explorer used to render ZK transfers as "unknown opcode" because the parser was Bitcoin-Core-flavoured and didn't speak our witness-v2. Refactored to detect the privacy opcode prefix early in the decoder, then route to a dedicated renderer that shows nullifiers, commitments, and proof byte length without ever trying to interpret the encrypted note ciphertext. Genesis scan was the surprise win:…