[codex] implement WASI poll_oneoff fd polling for moonrun

[peter-jerry-ye]

Summary

Implement focused WASI poll_oneoff support in moonrun for fd read/write readiness.

• Register poll_oneoff in the WASI imports.
• Reject clock subscriptions with NOTSUP to keep this implementation scoped.
• Add fd read/write readiness events, including stdin hangup flags and stdout/stderr write readiness.
• Use rustix for Unix stdin polling and byte availability.
• Handle Windows stdin readiness directly with windows-sys for console and pipe HANDLE semantics that Rustix does not cover.
• Preserve nonblocking stdin behavior by mapping WouldBlock through WASI AGAIN.

Validation

• cargo test --locked -p moonrun --test test test_moonrun_wasi_poll_oneoff -- --nocapture
• cargo test -p moonrun
• cargo clippy -p moonrun --all-targets
• cargo fmt --check
• cargo check -p moonrun --target x86_64-pc-windows-msvc
• git diff --check

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/moonrun/src/wasi_api.rs	7% smaller
Cargo.lock	Unsupported file format
crates/moonrun/Cargo.toml	Unsupported file format
crates/moonrun/tests/test.rs	0% smaller