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-approvals.puml

@@ -0,0 +1,19 @@
+@startuml
+package "test-approvals" {
+  [TierImmutabilityTest]
+  [GrantSemanticsTest]
+  [ModeApprovalTest]
+  [ApprovalEngineEdgeCasesTest]
+  [NoExecutionCouplingTest]
+  [ApprovalTriggerTest]
+}
+
+[TierImmutabilityTest] --> [Tier]
+[GrantSemanticsTest] --> [DefaultApprovalEngine]
+[ModeApprovalTest] --> [DefaultApprovalEngine]
+[ApprovalEngineEdgeCasesTest] --> [DefaultApprovalEngine]
+[NoExecutionCouplingTest] --> [DefaultApprovalEngine]
+[ApprovalTriggerTest] --> [ApprovalTrigger]
+
+[Tests] --> [ApprovalFixtures] : test data
+@enduml