docs: add Arc Pay reference DApp — gasless stablecoin transfers on Arc Testnet

[osr21]

Summary

This PR adds documentation for Arc Pay — a peer-to-peer gasless stablecoin transfer DApp built on Arc Network Testnet — as a reference implementation for the Arc ecosystem.

Live app: https://245e1045-f370-4cc1-8fc0-1ba2bc475ce3-00-3pv56lyfqrmzj.picard.replit.dev/
Source: https://github.com/osr21/arc-pay
Public RPC gateway: https://245e1045-f370-4cc1-8fc0-1ba2bc475ce3-00-3pv56lyfqrmzj.picard.replit.dev/rpc

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What's Added

docs/ecosystem/arc-pay-reference-dapp.md — a full reference document covering:

• Gasless transfer flow using EIP-2612 permit + relayer (no ETH needed, no approval tx)
• Arc RPC Gateway — a public load-balanced JSON-RPC proxy built as a workaround for the missing CORS headers on rpc.testnet.arc.network (filed as #90)
• USDC decimal handling — the 18 vs 6 decimal distinction between native gas usage and ERC-20 context (filed as #91)
• Developer findings — links to 5 issues filed from real development experience building on Arc Testnet
• Architecture diagram, stack breakdown, and wallet setup instructions

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Why This Is Relevant to Arc

Arc's core differentiator — USDC as the native gas token — enables gasless DApp UX that is not possible on other EVM chains. Arc Pay demonstrates this end-to-end with a production implementation. Including it in the Arc Node docs gives:

1. A concrete example of what gasless UX looks like on Arc using EIP-2612 permit
2. A public RPC endpoint that other developers can use (the gateway is open for read calls)
3. Documented friction points that future developers will hit, with workarounds already solved
4. A reference for the EIP-2612 permit flow on USDC — currently undocumented in Arc resources (issue #93)

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File Added

docs/ecosystem/arc-pay-reference-dapp.md

Happy to restructure the file path or adjust content to fit whatever conventions the Arc team prefers for ecosystem documentation.