Function apply_text_case 

pub fn apply_text_case(text: &str, case: TextCase) -> String

Apply a text-case transform to a single plain-text segment.

This function handles the core casing logic for a single string. For structured titles with subtitles, use apply_to_structured_parts.

.nocase-protected spans are handled at the Djot rendering layer, not here — this function operates on already-resolved text segments.