ecosystem: add x402s — state channel payments for agents and APIs

[pr0toshi]

Adds x402s to the ecosystem page under Infrastructure & Tooling, along with the statechannel scheme spec docs.

x402s is the reference implementation of the statechannel payment scheme for x402. Agents open a funded channel once, pay thousands of API calls off-chain via signed state updates, and settle on-chain only when the channel closes.
Reference implementation: github.com/Keychain-Inc/x402s

Description

Adds x402s to the ecosystem page under Infrastructure & Tooling, along with the statechannel scheme spec docs.
x402s is the reference implementation of the statechannel payment scheme for x402. Agents open a funded channel once, pay thousands of API calls off-chain via signed state updates, and settle on-chain only when the channel closes.
Components:
X402StateChannel.sol — on-chain dispute contract (deterministic CREATE2)
Hub server — quote/issue/settlement workflows with configurable fees
Agent SDK + CLI — discover offers, open channels, make paid API calls
Payee server template — protect any API route with 402 + statechannel offers
Challenge watcher — monitors and disputes stale on-chain closes

Ecosystem entry:

typescript/site/app/ecosystem/partners-data/x402s/metadata.json
typescript/site/public/logos/x402s.png

Spec docs:

specs/schemes/statechannel/scheme_statechannel.md — scheme overview, trust model, security requirements
specs/schemes/statechannel/scheme_statechannel_evm.md — EVM implementation: EIP-712 signing, contract interface, verification, settlement, streaming
docs/extensions/state-channels.mdx — user-facing docs page
docs/docs.json — nav entry (+1 line)

Reference implementation: github.com/Keychain-Inc/x402s

Tests

No runtime code changes — this PR adds only static content (spec markdown, docs page, ecosystem metadata, logo). Verified:

docs.json is valid JSON after nav entry insertion
All file paths match the existing ecosystem and specs directory conventions
Logo is PNG, metadata.json matches the schema used by other ecosystem entries

Checklist

• [ x] I have formatted and linted my code
• [ x] All new and existing tests pass
• [ x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• [ x] I added a changelog fragment for user-facing changes (docs-only changes can skip)

[cb-heimdall]

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