Improve save/load docs and VT slot preview flow

[han4wluc]

Summary

• add save/load requirements and integration docs, and update the RouteEngine action docs
• allow save slot integrations to inject thumbnail image and timestamp overrides through the action payload
• replace the old multi-slot VT with a reading-screen layered save flow, and capture thumbnails from the story container

Validation

• bun run vitest run spec/createEffectsHandler.routeGraphicsEvents.test.js spec/projectDataSchema.test.js
• rtgl vt screenshot --wait-event vt:ready --item save/dynamic-slot-selector-test.yaml
• rtgl vt report --item save/dynamic-slot-selector-test.yaml --diff-threshold 0.8