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-projections.puml

@@ -0,0 +1,8 @@
+@startuml
+[Test] -> [EventFixtures] : create event sequence
+[Test] -> [Projection] : project(events)
+[Projection] -> [Reducer] : reduce(state, event)
+[Reducer] --> [Projection] : new state
+[Projection] --> [Test] : final state
+[Test] --> [Assertions] : verify fields
+@enduml