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-contracts.puml

@@ -0,0 +1,6 @@
+@startuml
+[Contract Tests] -> [Domain Types] : construct
+[Contract Tests] -> [kotlinx.serialization] : encode/decode
+[Domain Types] --> [Contract Tests] : value
+[Contract Tests] --> [Assertions] : verify contract
+@enduml