docs: add Vellar x402 facilitator to Stellar ecosystem

docs/build/agentic-payments/x402/README.mdx

@@ -95,6 +95,35 @@ This version supports x402 v2 specification. For x402 v1 support, please use a p
 
 :::
 
+### Community facilitators
+
+The following facilitators are built and operated by the community. They are not
+maintained by SDF and are at varying stages of readiness, so check each project's
+current status before depending on it.
+
+#### Vellar Facilitator
+
+[Vellar](https://github.com/Vellar-Wallet/vellar-facilitator) is an Apache-2.0
+x402 facilitator that pairs the standard `/verify`, `/settle`, and `/supported`
+endpoints with a **Bazaar discovery layer**: a resource is added to a searchable
+catalog the first time a payment settles for it, so sellers are listed without a
+separate registration step. It also exposes that catalog to AI agents over
+[MCP](https://modelcontextprotocol.io), and sponsors the network fee on
+settlement, so buyers hold only the payment asset and no XLM.
+
+- **Facilitator**: `https://vellar-facilitator.onrender.com`
+- **Source**: https://github.com/Vellar-Wallet/vellar-facilitator
+- **Docs**: https://docs.vellar.xyz
+
+:::note
+
+Vellar runs on **Testnet only** and is pre-production. A mainnet deployment is
+gated on an external security review that has not yet been completed. The hosted
+instance runs on a free tier, so the first request after an idle period can take
+up to a minute.
+
+:::
+
 ## Examples
 
 - **x402 on Stellar (Stellar repo)** — Tools, examples, and references for the x402 protocol on Stellar. Use this as the canonical source for Stellar-specific x402 demos and tooling. [View on GitHub](https://github.com/stellar/x402-stellar)