Callbacks

A request stores a callback program and at most 128 bytes of callback data. The default pubkey means no callback.

fulfill writes CallbackExecuted, then CPIs into that program with no accounts. The payload is the Anchor callback discriminator, the 64-byte output, a little-endian length, and the stored bytes. If the callee returns an error, the transaction reverts. There is no retry. ADR 0006 is the constraint record.