Expose browser materializer contract

[chubes4]

Summary

• Adds wp-codebox/create-browser-materializer-contract so parent products can request the browser runner recipe/materialization contract without calling the browser session ability a second time.
• Reuses the existing browser Playground session builder to preserve the current recipe, materialization, authorization, and runnable context shape.
• Extends the WordPress plugin smoke to prove the new contract matches the recipe/materialization/authorization shape currently extracted from a duplicate session.

Fixes #500.

Tests

• php tests/smoke-wordpress-plugin.php
• npm run build
• npm run wordpress-plugin-smoke
• npm run recipe-browser-smoke
• npm run browser-runtime-operation-smoke
• npm run command-registry-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implemented the recipe-only browser materializer contract API, updated smoke coverage, and ran verification under Chris's review.