Add article: Generative UI as a runtime contract

[imaanmulji]

/claim #7

Summary

• Adds a developer-focused explainer for issue Written Content: What Is Generative UI? (And Why Text Output Is No Longer Enough) #7, "What Is Generative UI?"
• Frames generative UI as a runtime contract between component vocabulary, model output, renderer, state, actions, and guardrails.
• Covers where text output fails, how composition differs from arbitrary model-generated code, what developers still own, and where OpenUI fits naturally.

Validation

• npx --yes markdownlint-cli "Articles\Generative UI Is a Runtime Contract.md"
• git diff --check

Disclosure: AI-assisted with Codex; reviewed and linted before submission.