Vector store integrations: one package per store, none bundled in core

# Vector store packages

| Store | Package |
|---|---|
| Chroma | langchain-chroma |
| Pinecone | langchain-pinecone |
| Qdrant | langchain-qdrant |
| Weaviate | langchain-weaviate |
| MongoDB Atlas | langchain-mongodb |
| Elasticsearch | langchain-elasticsearch |
| Redis | langchain-redis |
| Postgres / pgvector | langchain-postgres |

```
pip install -U langchain-pinecone   # example
```

## Rules

- There is no vector store inside the `langchain` package. If docs or old code import a store from `langchain.vectorstores`, that path is legacy; the store now lives in its own `langchain-[store]` package.
- Pin the store package alongside langchain-core. Store integrations lag core releases, and a major bump on either side is the usual cause of weird serialization errors.
- The full provider list with Python and JS package names is on the integrations providers page; check there before guessing a package name.