Add SmartFlow Observatory x402 repositories to the x402 ecosystem

[smartflowproai-lang]

This adds 7 repositories from the SmartFlow Observatory org to the existing x402 ecosystem. All are original (no forks), public, and active.

• x402-attribution-ref: attribution evidence envelope spec + conformance vectors (referenced as an adopter case in Proposal: standardize facilitator attribution for x402 settlements x402-foundation/x402#2335)
• x402-b20-exact: draft B20 AssetTransferMethod for the x402 exact scheme (gasless settlement on Base)
• x402-endpoint-validator: CI validator for x402 endpoint conformance (reachability, well-known manifest, 402 body)
• mapper-mcp: MCP server exposing an x402 endpoint catalogue (58,800+ endpoints)
• x402-saas-starter: pay-per-download starter using x402 micropayments on Base
• quorum: multi-agent mesh paying itself in x402 micropayments, anchoring on Base mainnet (ETHGlobal Open Agents 2026 submission)
• solana-x402-scanner: x402 endpoint scanner for Solana mainnet (already listed under Base/Solana/Ethereum, adding it to x402 as well)