docs: add module documentation in AsciiDoc with PlantUML diagrams

Generated by per-group subagents covering all 52 modules across:
core (18), infrastructure (10), apps (5), interfaces (3),
plugins (7), testing (9)

Documentation format:
- AsciiDoc (.adoc) files in docs/modules/<group>/
- PlantUML (.puml) files in docs/diagrams/
- .adoc files reference diagrams via include:: directives

Each doc covers: purpose, responsibilities, non-responsibilities,
key types, event flow, integration points, invariants, PlantUML
diagram, known issues, and open questions.

docs/diagrams/test-transitions.puml

@@ -0,0 +1,7 @@
+@startuml
+[Test] -> [WorkflowFixtures] : graph
+[Test] -> [TransitionFixtures] : condition evaluator
+[Test] -> [DefaultTransitionResolver] : resolve(from, graph)
+[DefaultTransitionResolver] --> [Test] : next stage
+[Test] --> [Assertions]
+@enduml