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

[kratz00]

What is meant be "Concurrent receives from different calls" - there should be only one producer per context, or?

[kratz00]

What happens if there never is a "Next" message - will it block forever?

[tkilias]

this actual show another problem we cant stop to wait because, the split happens on the worker side, that we can add a block timeout we need to move the split into background thread. this has another consequence we need another mechanism to indicate send permission to the worker. the bg threads budget to the worker thread as long the worker thread still has budget it is allowed to send the current batch. the bg thread takes care splitting it using real remaining.