fix: render placeholder-free number format sections

[developer0hye]

Summary

• render selected number-format sections that contain only literals
• handle escaped and empty zero sections without changing numeric/date paths
• add an exact regression for the office2pdf fixture format
• apply current rustfmt output to two pre-existing lines so this branch satisfies its CI formatter gate

Validation

• red: the new zero-section regression returned 0 instead of -
• green: 42 unit tests passed
• green: 97 integration tests passed
• cargo clippy -- -D warnings
• cargo fmt --all -- --check

Upstream counterpart: MathNya#361

Downstream issue: developer0hye/office2pdf#1262