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/modules/plugins/plugin-validators.adoc

@@ -0,0 +1,48 @@
+= plugin-validators
+
+== purpose
+
+No source files present. This directory is reserved for external validation plugins that implement custom validation logic beyond the built-in validators (e.g., artifact payload validation). Currently a structural placeholder.
+
+== responsibilities
+
+* Declares the module structure for future validator plugin definitions
+
+== non-responsibilities
+
+* This module has no source code, types, or runtime behavior
+
+== key types
+
+*No types defined.*
+
+== event flow
+
+None. Module has no source files.
+
+== integration points
+
+* No current dependencies or dependents
+
+== invariants
+
+* Not applicable — module is empty
+
+== PlantUML diagram
+
+[plantuml, plugin-validators, "png"]
+----
+include::../../diagrams/plugin-validators.puml[]
+----
+
+
+
+
+== known issues
+
+* This module contains no implementation — it is an empty placeholder
+
+== open questions
+
+* Will validator plugins conform to the `Validator` functional interface from `:core:validation`?
+* Should validators be loaded at startup or discovered dynamically?