WIP [no ci]

- fix marking repeated headers/footers as artifacts
- fix table row grouping with empty cells

crates/typst-library/src/model/table.rs

@@ -870,13 +870,17 @@ cast! {
     "header" => Self::Header(NonZeroU32::ONE, TableHeaderScope::default()),
     "footer" => Self::Footer,
     "data" => Self::Data,
-    // FIXME: this is a little bit implicit.
-    // Make it more obvious that this should be a header.
     mut dict: Dict => {
-        let level = dict.take("level")?.cast::<Option<_>>()?.unwrap_or(NonZeroU32::ONE);
-        let scope = dict.take("scope")?.cast::<Option<_>>()?.unwrap_or_default();
-        dict.finish(&["amount", "all"])?;
-        Self::Header(level, scope)
+        // TODO: have a `pdf.header` function instead?
+        #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash, Cast)]
+        enum HeaderKind {
+            Header,
+        }
+        dict.take("kind")?.cast::<HeaderKind>()?;
+        let level = dict.take("level").ok().map(|v| v.cast()).transpose()?;
+        let scope = dict.take("scope").ok().map(|v| v.cast()).transpose()?;
+        dict.finish(&["kind", "level", "scope"])?;
+        Self::Header(level.unwrap_or(NonZeroU32::ONE), scope.unwrap_or_default())
     },
 }