v0.1.0 of on_device_rag

A lightweight, pure-Dart on-device Retrieval-Augmented Generation (RAG) engine for Flutter and Dart: chunk text, embed it, run vector similarity search, and build grounded prompts — fully offline, bri [...] Changelog excerpt: - Initial release. - RagEngineorchestrator with index, retrieveand ask. - TextChunkerfor overlapping character-window chunking. - EmbeddingModelinterface plus zero-dependency HashingEmbeddingModel. - VectorStoreinterface plus InMemoryVectorStorewith JSON persistence. - LanguageModelinterface for pluggable on-device or remote generation. - PromptBuilderwith a grounded default template and VectorMathhelpers.