Add progressive rendering reliability article

[tuanadr]

Summary

• adds an issue Written Content: OpenUI's React Renderer Explained: How Progressive Hydration Works with Streamed Model Output #3 article on progressive rendering as a reliability contract for streamed generated UI
• explains the pipeline from library and prompt to streamed OpenUI Lang, parser reruns, forward references, structured errors, tool providers, and onAction
• focuses on component-design implications for streaming: predictable roots, smaller .ref units, local loading states, and safe action boundaries

Validation

• git diff --check
• npx --yes markdownlint-cli "Articles/Progressive Rendering Is a Reliability Contract.md" --disable MD013

Disclosure

Drafting and editing were AI-assisted. I checked the article against current OpenUI docs for the overview, renderer behavior, toolProvider, onError, onAction, and component-library patterns, and I am responsible for the final wording and technical accuracy.

Related: #3