Function merge_meta 

pub fn merge_meta(
    base: &HashMap<String, Value>,
    update: &HashMap<String, Value>,
    merge_special: bool,
) -> HashMap<String, Value>

Merge update into base, matching Go’s MergeMeta logic.

• Keys ending in :* with truthy value clear the section.
• null values delete the key.
• If merge_special is false, keys starting with display: are skipped.