Function is_append_compatible 

pub fn is_append_compatible(
    older: &LayoutManifest,
    newer: &LayoutManifest,
) -> bool

Check if two layout manifests are append-compatible.

Returns true if newer is a strict superset of older:

• Same discriminator
• newer.version > older.version
• newer.total_size >= older.total_size
• Different layout IDs (proving the change)