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/infra-artifacts-cas.puml

@@ -0,0 +1,36 @@
+@startuml
+!theme plain
+package "infra-artifacts-cas" {
+  class CasArtifactStore
+  class CasConfig
+  interface ArtifactIndex
+  class SqliteArtifactIndex
+  class SegmentLayout
+  class SegmentWriter
+  class SegmentReader
+  class Location
+  class TailScanner
+  class LivenessScanner
+  class Compactor
+  class Evictor
+  class DefaultMaterializingArtifactWriter
+}
+interface ArtifactStore <<core:artifactstore>>
+interface MaterializingArtifactWriter <<core:artifacts>>
+ArtifactStore <|.. CasArtifactStore
+MaterializingArtifactWriter <|.. DefaultMaterializingArtifactWriter
+CasArtifactStore *--> CasConfig
+CasArtifactStore *--> ArtifactIndex
+CasArtifactStore *--> SegmentWriter
+CasArtifactStore *--> SegmentReader
+ArtifactIndex <|.. SqliteArtifactIndex
+CasArtifactStore --> TailScanner : recover()
+CasArtifactStore --> Compactor : compact()
+CasArtifactStore --> Evictor : evict()
+Compactor --> LivenessScanner
+Evictor --> LivenessScanner
+TailScanner --> ArtifactIndex : uses
+TailScanner --> SegmentLayout : uses
+SegmentWriter --> SegmentLayout : uses
+SegmentReader --> SegmentLayout : uses
+@enduml