Initial release of the core server-state engine.

• QueryClient, Query<T>, Mutation<T,V,C>, InfiniteQuery<T,P>.
• Deduplication, caching (staleTime/cacheTime), invalidation, garbage collection.
• Retry with exponential backoff + jitter.
• Optimistic updates with snapshot/rollback.
• Normalized QueryError + pluggable ErrorMapper.
• Single-flight auth-refresh coordination.
• Transport-agnostic HttpClientAdapter; persistence interfaces.
• Unit test suite covering the engine's core guarantees.

See ROADMAP.md for what's planned next (Dio/Riverpod/GetX/persistence adapters, offline mutation queue, DevTools UI).