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-fixtures.puml

@@ -0,0 +1,20 @@
+@startuml
+package "test-fixtures" {
+  [EventFixtures]
+  [InferenceFixtures]
+  [ApprovalFixtures]
+  [WorkflowFixtures]
+  [TransitionFixtures]
+  [CycleFixtures]
+  [ContextFixtures]
+  [MockInferenceProvider]
+  [MockTokenizer]
+  [EchoTool]
+  [FailingTool]
+  [RejectedTool]
+  [MockEventReplayer]
+  [MockSessionRepository]
+}
+
+[Testing Modules] --> [test-fixtures] : reuse in tests
+@enduml