docs(specs): normative spec of Diagram.expand, with a traversal shape catalog

mkdocs.yaml

@@ -121,6 +121,7 @@ nav:
               - Primary Keys: reference/specs/primary-keys.md
               - Fetch API: reference/specs/fetch-api.md
               - Diagram: reference/specs/diagram.md
+              - Traversal Shapes: reference/specs/traversal-shapes.md
           - Type System:
               - Types: reference/specs/type-system.md
               - Codec API: reference/specs/codec-api.md

src/reference/specs/index.md

@@ -101,6 +101,7 @@ Each specification follows a consistent structure:
 | [Primary Keys](primary-keys.md) | Table Declaration | [Design Primary Keys](../../how-to/design-primary-keys.md/) | [Entity Integrity](../../explanation/entity-integrity.md/) |
 | [Fetch API](fetch-api.md) | Query Operators | [Fetch Results](../../how-to/fetch-results.md/) | — |
 | [Diagram](diagram.md) | Table Declaration | [Read Diagrams](../../how-to/read-diagrams.ipynb/) | — |
+| [Traversal Shape Catalog](traversal-shapes.md) | Diagram, Master-Part | [Delete Data](../../how-to/delete-data.md/) | — |
 
 **Key concepts:** Restriction (`&`, `-`), projection (`.proj()`), join (`*`), aggregation (`.aggr()`), union, universal set (`U()`), attribute lineage, schema visualization