refactor: trim cascade diagram to seed + descendants subgraph

cascade() now removes non-descendant nodes from the returned Diagram,
so the graph itself defines the delete scope. This eliminates the
redundant _cascade_restrictions membership check in delete() — it
simply walks all non-alias nodes in the trimmed graph.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -380,6 +380,15 @@ def cascade(self, table_expr, part_integrity="enforce"):
         result._restriction_attrs[node] = set(table_expr.restriction_attributes)
         # Propagate downstream
         result._propagate_restrictions(node, mode="cascade", part_integrity=part_integrity)
+        # Trim graph to cascade subgraph: only restricted tables
+        # (seed + descendants) plus alias nodes connecting them.
+        keep = set(result._cascade_restrictions)
+        for alias in (n for n in result.nodes() if n.isdigit()):
+            if set(result.predecessors(alias)) & keep and set(result.successors(alias)) & keep:
+                keep.add(alias)
+        result.remove_nodes_from(set(result.nodes()) - keep)
+        result.nodes_to_show &= keep
+        result._expanded_nodes &= keep
         return result
 
     def _restricted_table(self, node):
@@ -625,9 +634,9 @@ def delete(self, transaction=True, prompt=None, dry_run=False):
 
         conn = self._connection
 
-        # Get non-alias nodes with restrictions in topological order
+        # Get non-alias nodes in topological order (graph is already trimmed by cascade())
         all_sorted = topo_sort(self)
-        tables = [t for t in all_sorted if not t.isdigit() and t in self._cascade_restrictions]
+        tables = [t for t in all_sorted if not t.isdigit()]
 
         # Preview
         if prompt: