Fix literal-only zero number format sections

[developer0hye]

Summary

• render selected number-format sections that contain only literals
• support quoted text, backslash escapes, spacing markers, fill characters, and empty zero sections
• preserve the existing date and numeric formatting paths whenever a value placeholder is present

Problem

A three-section format such as #,##0_);[Red]\(#,##0\);\-\ \ selects the third section for zero, but the formatter currently falls through to numeric formatting and returns 0 instead of the declared - literal.

Testing

• cargo test
• cargo clippy -- -D warnings
• cargo +nightly fmt --all -- --check

The width-independent string formatter represents *x with one fill character; it does not expand the character to a cell width.

[developer0hye]

CI note: the failing run is blocked by current baseline/toolchain drift rather than this change.

• build (1.88.0) resolves aes 0.9.3, which now requires Rust 1.89, because the library does not commit a lockfile.
• formatting reports current-nightly wrapping changes across existing files (date.rs, reader/driver.rs, writer/streaming_writer.rs, and others). The reported number_format.rs hunk is also an unchanged pre-existing comment.
• The exact PR head passed cargo test, cargo clippy -- -D warnings, and cargo +nightly fmt --all -- --check locally before push; the run logs show dependency/nightly changes after that validation.

No production failure from the literal-only section implementation appears in the CI log.