Deduplicate output install pipeline

[ryanfowler]

Summary

• Extracted shared DownloadTemp setup and install/commit logic in src/output/mod.rs
• Kept sync and async writers different only in how bytes are copied into the temp file
• Added regression tests covering target-race handling for both sync and async output writes

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1