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/core-artifacts.puml

@@ -0,0 +1,57 @@
+@startuml
+interface Artifact
+interface ArtifactReducer
+interface ArtifactKind
+interface ArtifactKindRegistry
+interface ArtifactRepository
+interface MaterializingArtifactWriter
+interface Projection
+
+class DefaultArtifactReducer
+class DefaultArtifactKindRegistry
+class ArtifactProjector
+
+class ArtifactState
+class ArtifactLineage
+class ArtifactRelationship
+class FileWrittenKind
+class ProcessResultKind
+class FileWrittenPayload
+class FileWrittenArtifact
+class ProcessResultArtifact
+class TypedArtifactSlot
+
+ArtifactReducer <|.. DefaultArtifactReducer
+ArtifactKindRegistry <|.. DefaultArtifactKindRegistry
+ArtifactKind <|.. FileWrittenKind
+ArtifactKind <|.. ProcessResultKind
+Projection <|.. ArtifactProjector
+ArtifactProjector --> ArtifactReducer
+
+DefaultArtifactKindRegistry --> ArtifactKind
+DefaultArtifactReducer ..> ArtifactState
+ArtifactState --> ArtifactLineage
+ArtifactLineage --> "relationships" ArtifactRelationship
+
+FileWrittenKind ..> FileWrittenPayload
+FileWrittenKind ..> FileWrittenArtifact
+ProcessResultKind ..> ProcessResultArtifact
+
+package "core:events" <<External>> {
+  [ArtifactCreatedEvent]
+  [ArtifactValidatingEvent]
+  [ArtifactValidatedEvent]
+  [ArtifactRejectedEvent]
+  [ArtifactSupersededEvent]
+  [ArtifactArchivedEvent]
+  [ArtifactRelationshipAddedEvent]
+}
+package "core:artifacts-store" <<External>> {
+  [ArtifactStore]
+}
+
+DefaultArtifactReducer ..> "core:events"
+ArtifactProjector ..> "core:events"
+MaterializingArtifactWriter ..> "core:artifacts-store"
+
+@enduml