Replace the one-image-per-interaction model with scenes that hold multiple
dialogue beats. The image regenerates only on scene-change actions; tapping
through beats and in-scene choices are instant and zero-network.
Squashed from #2:
- feat: scene/beat architecture — decouple dialogue from image generation
- fix: harden LLM-output parsing, prefetch lifecycle, and typewriter (PR review)
- fix: dedupe beat ids; fallback narration on empty insert-beat (PR review #2)
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Zonghao Yuan