ragleap-rag (Java) v0.6.0

Adds FaissBackend — a second vector backend alongside pgvector. Pure-Java brute-force exact search (no native FAISS library, no JNI), in-memory index, SQLite metadata sidecar, custom documented persistence format. 12 new tests, all passing. Published to Maven Central: io.github.antonyrag:ragleap-rag:0.6.0