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/core/core-observability.adoc

@@ -0,0 +1,51 @@
+= core-observability
+
+== purpose
+
+Placeholder module reserved for observability primitives (metrics, structured logging, tracing). Currently contains no source files — the module is a Gradle subproject with no implementation.
+
+== responsibilities
+
+* Intended: define observability contracts (metric types, span interfaces)
+* Intended: provide base abstractions used by `infra:telemetry`
+
+== non-responsibilities
+
+* Does not implement telemetry infrastructure — that belongs to `infra:telemetry`
+* Does not collect runtime metrics directly
+* Does not manage log output destinations
+
+== key types
+
+*None.* The module has no source files.
+
+== event flow
+
+*None.* No events are consumed or emitted. This is a placeholder.
+
+== integration points
+
+No integration points. The module has no dependencies beyond `core:events`.
+
+== invariants
+
+None — module is empty.
+
+== PlantUML diagram
+
+[plantuml, core-observability, "png"]
+----
+include::../../diagrams/core-observability.puml[]
+----
+
+
+
+
+== known issues
+
+Module is a placeholder — no observability contract exists.
+
+== open questions
+
+* Which metrics framework will be used (Micrometer, OpenTelemetry, custom)?
+* Will observability be event-sourced or side-channel?