Function append 

pub fn append(body: &str, entry: &Entry<'_>, mode: Mode) -> String

Append entry to body (the part after frontmatter), returning the new body.

• Reflect: a new date appends ## DATE; a repeat date nests ### HH:MM.
• Journal: every turn appends ## HH:MM (the file is already one date); a --- rule separates same-day entries. The blank line BEFORE the rule is required — --- directly under text is a setext heading underline, not a divider.