feat: prototype graph zone collapse

docs/graph-explorer-contract.md

@@ -195,6 +195,13 @@ such as `zoneBoundaries`, `zoneGrouping`, and `zoneDefinitionSet`, but saved
 profiles should prefer the nested object so future zone definition sets and
 presentation preferences can be restored without another state migration.
 
+Zone collapse is a view-only operation. A collapsed zone should hide its visible
+member nodes, replace them with a synthetic zone node, and draw synthetic
+boundary edges from that zone node to visible external neighbors. Internal edges
+are summarized on the zone node rather than rendered. Expanding the zone removes
+the synthetic elements and restores the original graph elements without
+changing the underlying payload.
+
 ## Repo-Scoping Compatibility
 
 Repo-scoping can adapt without a rewrite because its current graph payload