Add OpenUI React renderer progressive hydration article

[starlord-94]

Summary

• Adds a source-grounded article for issue Written Content: OpenUI's React Renderer Explained: How Progressive Hydration Works with Streamed Model Output #3 on OpenUI's React renderer and streamed model output.
• Explains the current path from streamed text through createStreamingParser, autoClose, materializeValue, useOpenUIState, and Renderer.
• Clarifies that this is progressive UI materialization/rendering rather than React SSR hydration, and covers query/mutation deferral plus last-good render fallback behavior.
• Includes plain-text diagrams and code excerpts tied to the current OpenUI source.

Validation

• Checked the article against OpenUI source commit 9c60c4baf43083bab9cd9ae189d42658c959fec7.
• git diff --cached --check
• Word count: 2,512 words.

For #3