Capture editor iframe presentation coverage

[chubes4]

Summary

• capture stylesheet URLs and generated presentation identities from the Gutenberg editor canvas
• wait for canvas/style readiness while excluding unrelated iframes and preserving non-iframe editor compatibility
• cover delayed insertion, late styles, and canvas replacement in browser tests

Fixes #2252.

Verification

• npm run test:editor-actions
• npm run test:browser-routed-command-security
• npm run build

AI assistance

OpenAI gpt-5.6-sol was used through OpenCode to inspect the editor runtime, implement the capture contract, and add regression coverage. Chris Huber reviewed and is responsible for the changes.