Function copy_file 

pub fn copy_file(
    src: &Path,
    dst: &Path,
    mode: ReflinkMode,
) -> Result<CopyOutcome, WorktreeError>

Copy a single file from src to dst respecting the given ReflinkMode.

• Required: use CoW only; return ReflinkNotSupported if the FS doesn’t support it.
• Preferred (default): try CoW, fall back to standard copy.
• Disabled: always use standard copy.

Returns the CopyOutcome describing what actually happened.