feat: model FableLoom audience participation

[atomantic]

Summary

• add explicit FableLoom audience roles for helper-guided and protagonist-controlled stories
• model each helper scene communication state so decisions unlock only while the configured in-story medium is connected
• carry the participation contract through persistence, prompts, generation, validation, playback, editing, and series creation

Test plan

• npm run build --prefix client
• npm test
• focused FableLoom server and client suites