docs: add Vellar x402 facilitator to Stellar ecosystem

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

@@ -59,7 +59,7 @@ x402 on Stellar supports any [SEP-41](https://stellar.org/protocol/sep-41) compl
 
 ## x402 Facilitators
 
-You can use a facilitator to verify and settle x402 payments. Two options are available for Stellar:
+You can use a facilitator to verify and settle x402 payments. Two production implementations are available for Stellar, plus one community-run pre-production option listed at the end of this section:
 
 ### Coinbase x402 Facilitator
 
@@ -95,6 +95,42 @@ 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**: when a payment settles and its
+payload carries the Bazaar discovery extension, the resource is added to a
+searchable catalog, so sellers that use the extension are listed without a
+separate registration step. Payments without the extension settle normally but
+are not cataloged. The catalog is also reachable by AI agents through an
+[MCP](https://modelcontextprotocol.io) discovery server (`src/mcp.ts` in the
+repository), which is a separate stdio process you run locally and point at the
+facilitator URL; the hosted instance does not expose an MCP endpoint itself. The
+facilitator 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; there is no mainnet
+instance. The pre-mainnet security review of the facilitator is complete, with
+findings tracked to closure. A mainnet release is additionally gated on a
+separate audit of the spending-limit policy contract, which has not yet been
+run. 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)