fix(parser): reject invalid zippy output mappings

[ShiroKSH]

Summary

Reject malformed no-erase and single-output zippychord mappings with parser errors.

Previously, mappings without required output values indexed an empty list and panicked during configuration validation. A no-erase mapping with extra values was also accepted instead of rejected.

Validation

• cargo test --all --no-default-features
• cargo test --all
• cargo clippy --all --no-default-features -- -D warnings
• cargo clippy --all -- -D warnings
• cargo fmt --all --check

[ShiroKSH]

@jtroo We'd appreciate it if you could take a look at this pull request when you have a chance.

[jtroo]

Thanks!