v2.1.0

**Improvements:**
- **Client:** Requests now carry three surface-identity headers so the platform can tell which product made a call. `X-Mem0-Source` names the surface and `X-Application` the host app it runs inside, both set-once so a wrapper that already declared its identity keeps it. `X-Mem0-Client` is append-only and carries `name/version` per layer, outermost first, so a plugin calling this SDK reports the whole chain rather than only the last speaker. `MEM0_SOURCE`, `MEM0_APPLICATION` and `MEM0_CLIENT_STACK` set them from the environment for wrappers that cannot pass options ([#7326](https://github.com/mem0ai/mem0/pull/7326))
- **Client:** The client stack is bounded by dropping whole entries rather than slicing characters, and this SDK's own entry is the reserved one. Truncating the joined string could sever an identifier mid-name and the platform parsed the fragment as a real client ([#7326](https://github.com/mem0ai/mem0/pull/7326))