Document v2 runner architecture

[tkilias]

Summary

• add documentation-only v2 runner architecture
• define the private C++ implementation, stable C ABI, and header-only C++ facade
• document socket acceptance, reusable worker-pool dispatch, context creation, ownership, shutdown, and ABI rules
• add Mermaid source diagrams with rendered SVGs for layering, components, lifecycle, descriptor ownership, and the ABI boundary

Validation

• all runner Markdown links resolve
• all Mermaid source/render pairs exist
• Mermaid diagrams render successfully
• git diff --check passes