docs: add ADR-012 and MessagePack serialization guidelines

- Update design.md with MessagePack component requirements
- Add ADR-012 to architecture.md regarding public type requirements
- Add MessagePackAnalyzer to OECS.csproj for compile-time validation

docs/architecture.md

@@ -334,3 +334,32 @@ no parallel scheduling.
 
 - Consumers must be on .NET 8 or later.
 - Can use `ref` returns, `readonly struct`, and other modern C# features.
+
+---
+
+## ADR-012: Public Types Required for MessagePack Serialization
+
+**Status:** Accepted
+
+**Context:** Component types must be serializable by MessagePack-CSharp. The
+library imposes constraints on type design.
+
+**Decision:** Component types must be `public` and annotated with
+`[MessagePackObject]` and `[Key]` attributes. The `MessagePackAnalyzer` NuGet
+package is included for compile-time validation.
+
+**Rationale:**
+
+- MessagePack-CSharp requires public types for its dynamic formatter generation
+  and source-generated formatters.
+- `MessagePackAnalyzer` provides AOT-safe source-generated formatters (critical
+  for Unity IL2CPP) and catches misconfigured types at compile time.
+- Indexed integer keys (`[Key(0)]`) produce the fastest and most compact
+  serialization, which aligns with the ECS performance goal.
+
+**Consequences:**
+
+- Component authors cannot use `private` or `internal` types.
+- The `MessagePackAnalyzer` package is a compile-time dependency.
+- `[Key]` indices should be sequential starting from 0 to avoid null
+  placeholders in the binary output.