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-validation.puml

@@ -0,0 +1,60 @@
+@startuml
+interface Validator
+interface SemanticRule
+
+class ValidationPipeline
+class GraphValidator
+class TransitionValidator
+class SessionValidator
+class SemanticValidator
+class CyclePolicyBindingRule
+class ArtifactPayloadValidator
+class ApprovalTrigger
+
+class ValidationOutcome
+class ValidationReport
+class ValidationSection
+class ValidationIssue
+class ValidationContext
+
+package "core:transitions" <<External>> {
+  [WorkflowGraph]
+  [DetectedCycle]
+  [CyclePolicyBinding]
+  [TransitionEdge]
+}
+package "core:artifacts" <<External>> {
+  [FileWrittenArtifact]
+  [ProcessResultArtifact]
+}
+package "core:artifacts-store" <<External>> {
+  [ArtifactStore]
+}
+
+Validator <|.. GraphValidator
+Validator <|.. TransitionValidator
+Validator <|.. SessionValidator
+Validator <|.. SemanticValidator
+Validator <|.. ArtifactPayloadValidator
+SemanticValidator --> "rules" SemanticRule
+SemanticRule <|.. CyclePolicyBindingRule
+ValidationPipeline --> "validators" Validator
+ValidationPipeline --> ApprovalTrigger
+ValidationPipeline ..> ValidationOutcome
+ValidationOutcome --> ValidationReport
+ValidationOutcome ..> ApprovalRequest
+ValidationReport --> "sections" ValidationSection
+ValidationSection --> "issues" ValidationIssue
+ValidationIssue --> ValidationSeverity
+ValidationIssue --> ValidationLocation
+
+GraphValidator ..> "core:transitions"
+TransitionValidator ..> "core:transitions"
+SessionValidator ..> "core:sessions"
+CyclePolicyBindingRule ..> "core:transitions"
+ArtifactPayloadValidator ..> "core:artifacts"
+ArtifactPayloadValidator ..> "core:artifacts-store"
+ValidationContext ..> "core:transitions"
+ValidationContext ..> "core:sessions"
+
+@enduml