docs(c4): add cross-diagram stub convention note to header

The C4 standard doesn't define this pattern. Adding a one-sentence
explanation so readers unfamiliar with the project's rendering convention
understand what stub components outside System_Boundary blocks mean.

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

docs/architecture/c4-diagrams.md

@@ -1,6 +1,8 @@
 # Familienarchiv — C4 Architecture Diagrams
 
 > For domain terminology used in these diagrams, see [docs/GLOSSARY.md](../GLOSSARY.md).
+>
+> **Cross-diagram stubs:** Components placed outside a `System_Boundary` block with a "See diagram X" annotation are reference stubs — they represent a component fully defined in another sub-diagram and appear here only to show the cross-domain dependency without duplicating the full definition.
 
 ## Level 1 — System Context