feat: add interactive FableLoom camera-cut playback

[atomantic]

Summary

• model every FableLoom node as one camera cut with a shared movement vocabulary and manual selectors
• distinguish automatic cuts from looping viewer-decision nodes and validate their graph contracts
• preview complete episode sequences as text, storyboard images, or rendered video
• let the story writer choose node counts and document the interactive-video production model

Test plan

• cd server && npm test -- --run fableLoom lib/index.test.js
• cd client && npm test -- --run fableloom FableLoomStory
• cd client && npm run lint
• cd client && npm run build