Diagram spec: document the master-part entity cluster #298

Summary

reference/specs/diagram.md documents the tier shapes, the edge styles, the underline, and schema grouping, but not the master-part entity cluster that dj.Diagram has drawn since the 2.3 notation work (datajoint-python #1532 / #1534). A reader of the spec cannot learn why parts sit in a shaded group, why a part's label lacks its master's name, or how the group is laid out. These are the questions people ask when they meet the group in a figure.

What the renderer does (datajoint-python diagram.py)

• Grouping. Within a schema cluster, a master and its visible parts are enclosed in a nested, unlabeled cluster with a fill (entity_fill: #F3F5F8 light, #2A313D dark) and no border. Edges are drawn over it.
• Part labels. Inside the group, a part shows its own name only (Scan, not Acquisition.Scan), because the group carries the membership. A part shown without its master is labeled Master.Part.
• Layout. The master and its parts share a rank, so the horizontal reads as derivation and the vertical as containment. The exception is a part that depends on a sibling part, which is left off the rank so the chain within the group descends.
• Edges. A master→part edge follows the ordinary weight rule. A part almost always adds a key attribute, so the edge is thin (the spec says this under Edge Styles). The part is underlined when it introduces a dimension.

Proposed addition

A short "Master-part grouping" section after "Schema Grouping", covering the four points above. It should include one figure from a schema with a master and two parts, one of which depends on a sibling, and link to master-part.md for the semantics (one entity spread over rows, inserted in one transaction, deleted together).

Related: #299 (the master-part spec side), #249 (diagram-traversal algebra in the same spec, including the master↔part group rule for restriction).