Macro export_block 

macro_rules! export_block {
    ($ty:ty) => { ... };
}

Emit the wasm exports for a Block implementation.

The block’s state lives in a thread-local because the host instantiates one module per job and never shares it — so there is exactly one block instance per module instance, and no cross-job state that could leak between them.

Every export takes and returns usize rather than a packed integer. On wasm32 that lowers to i32 parameters and results; on a 64-bit guest the same source compiles to i64 with nothing here changing.