feat(host): submit async functions through generic drivers

[fffonion]

Summary

• generate generic async host submissions from native async signatures
• route futures through host-owned drivers without a dedicated edge macro path

Stack

Layer 8 of 14
Previous: #18
Next: #20

Validation

• cargo test --locked -p pd-host-function
• cargo test --locked --all-features --lib async_host
• cargo test --locked --all-features --test runtime_host_tests

[fffonion]

Superseded by #18. Generic async host drivers are now part of the capability and async-execution layer.