Function diff_materialized_views 

pub fn diff_materialized_views(
    target_mvs: &[MaterializedView],
    source_mvs: &[MaterializedView],
    out: &mut ChangeSet,
    managed_roles: &BTreeSet<Identifier>,
)

Diff target.materialized_views (live DB) against source.materialized_views (desired).

Emits:

• MvChange::Create for MVs present only in source.
• MvChange::Drop for MVs present only in target (Safe: MVs are derived).
• MvChange::ReplaceBody when the canonical body hashes differ.
• MvChange::SetComment / MvChange::SetColumnComment for metadata.
• Change::AlterObjectOwner / grant changes when applicable.