feat(host): infer typed host call contracts from signatures

[fffonion]

Summary

• infer host binding kind, return schema, and suspension behavior from Rust signatures
• use VmResult<T> for VM errors and HostCallResult<T> for return-or-pending behavior
• remove BuiltinResult<T> and reject the undefined HostResult<T> wrapper across generation and macros

Stack

Layer 1 of 9
Base: master
Next: #13

Validation

• cargo test --locked -p pd-host-function
• cargo test --locked --test host_binding_generation_tests
• cargo test --locked --test builtins_tests