Guest-side library for writing cuttlefish proc-blocks.
Stub: the Block trait and the wasm export macro land here next.
A block is a state machine that the host drives — it returns commands and is
stepped, rather than calling the host and blocking. This crate exists to hide
that inversion behind an ordinary-looking Rust trait, so block authors do not
hand-write state transitions. See [cuttlefish_abi] for why control is
inverted in the first place.