Add software structure tree diagram, system design doc, integration tests, and review-set restructuring

docs/design/introduction.md

@@ -28,6 +28,31 @@ The following topics are out of scope:
 - Build pipeline configuration
 - Deployment and packaging
 
+## Software Structure
+
+The following tree shows how the ReqStream software items are organized across the system,
+subsystem, and unit levels:
+
+```text
+ReqStream (System)
+├── Command-Line Interface (Subsystem)
+│   └── Context (Unit)
+├── Program Orchestration (Subsystem)
+│   └── Program (Unit)
+├── Requirements File Processing (Subsystem)
+│   ├── Requirements (Unit)
+│   ├── Section (Unit)
+│   └── Requirement (Unit)
+├── Test Integration (Subsystem)
+│   └── TraceMatrix (Unit)
+├── Validation (Subsystem)
+│   └── Validation (Unit)
+└── Linting (Subsystem)
+    └── Linter (Unit)
+```
+
+Each unit is described in detail in its own chapter within this document.
+
 ## Document Conventions
 
 Throughout this document: