Use namespaces for IndexedDB persistence

[han4wluc]

Summary

• replace browser localStorage persistence with an IndexedDB adapter
• require a per-VN namespace for hydration and engine.init(...) so multiple VNs on one domain do not overwrite each other
• update the VT bootstrap and docs, and add persistence coverage for namespace isolation

Testing

• bun run vitest run spec/indexedDbPersistence.test.js spec/createEffectsHandler.routeGraphicsEvents.test.js spec/projectDataSchema.test.js spec/RouteEngine.lineCompletion.test.js spec/RouteEngine.rollbackRenderState.test.js
• bun run build
• bun run vt:generate
• Chromium VT verification for candidate/save/basic and candidate/save/dynamic-slot-selector-test