feat: support scene descriptions and rect layout elements

[han4wluc]

Summary

• add description support to scene state validation plus scene.create and scene.update
• persist scene descriptions in reducers and extend story/sequence tests to cover the new field
• add model validation support for rect layout elements including fill and border fields

Testing

• bunx vitest run tests/story-and-scenes.spec.yaml tests/command-sequences.test.js
• bunx prettier --check src/model.js tests/story-and-scenes.spec.yaml tests/command-sequences.test.js