Fix unit result variable mappings

[zakazaka95]

Summary

• discard zero-sized produced results before Cairo variable mappings are deduplicated
• keep the data-carrying result of x += value from being overwritten by its unit return value
• remove the downstream display workaround and add a focused regression test

Closes #128.

Validation

• cargo test
• cargo test --release
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps
• formatting and git diff --check