Fix async command sequence failure propagation - #4436

Summary

• return the worker result from Sequence::wait()
• propagate child spawn failures, nonzero exits, and worker panics to callers
• make WASM package generation fail when a generation step fails
• add focused sequence success and failure tests

This prevents a failed wasm-bindgen or wasm-opt step from being reported as a successful WASM build.

Testing

• cargo fmt --package cargo-run -- --check
• cargo test -p cargo-run