examples: switchboard_x402_client — Rust ↔ Python interop demo

[Pattermesh]

Adds `examples/switchboard_x402_client.rs` — an arka-based agent calling a paid endpoint served by Python switchboard's HTTP-402 middleware. Proves the wire format (`PaymentOffer` / `PaymentProof` JSON envelopes) is genuinely language-neutral: arka has no switchboard dependency, it just parses and emits the agreed shape.

What the example does

• Creates a fresh arka `Agent` on Base
• `GET endpoint` cold; expects `402 Payment Required` + `X-Payment-Required` header
• Parses the `accepts[]` envelope (matches `switchboard.x402_middleware.PaymentOffer`)
• Builds an `X-Payment-Proof` header (matches `switchboard.x402_middleware.PaymentProof`)
• Retries with the proof; expects `200`

The tx-hash is stubbed for the demo (real settlement would go through arka's wallet or switchboard's `AgentEscrow` contract via alloy). The point is the wire — bytes leaving the Rust process round-trip through the Python middleware's `PaymentProof.from_header()` without loss.

Why this matters

• The stack scene in switchboard's lab (web/agents-demo.html scene 09) names arka as a "consumer" of switchboard — this example backs that claim with runnable code.
• When the spec evolves (PQ envelope kcolbchain/switchboard#33, receipt aggregation #43, private x402 #44), this example is the canary that catches wire drift between the Rust and Python sides.
• The planned adversarial conformance harness (#45) will eventually run this as a cross-language fixture.

Tests

`cargo check --example switchboard_x402_client` passes clean (no warnings). The example is run-only (needs the Python middleware up); it's not part of the default test suite.

Other changes

• README.md examples table updated to include `switchboard_x402_client`

cc @abhicris

🤖 Generated with Claude Code

[abhicris]

Welcome to kcolbchain, @Pattermesh — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

What happens after your first merge

• You're listed on https://kcolbchain.com/contributors/ (opt-out anytime via data/contributors-optout.txt).
• At 2+ merged PRs you're a Regular; at 5+ you're a Fellow — Fellows are the pool we invite first for paid research, partner-org gigs, and grant-funded work. Details: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for what to pick up next? https://kcolbchain.com/invitations/ lists open help-wanted / good-first-issue across every kcolbchain repo.

Thanks for writing the code. We're building this to last.

[abhicris]

🤖 Audit verdict: safe

Example file demonstrating HTTP-402 payment interop with proper error handling, no hardcoded secrets, ephemeral wallet generation, and mocked transaction for demo purposes only.

Audited by the kcolbchain PR pipeline. See pipeline docs.

[abhicris]

Merged — thank you, @Pattermesh. Your first kcolbchain contribution is in. 🎉

A few things that unlocked with this merge:

• You'll appear on https://kcolbchain.com/contributors/ on the next daily rebuild. Opt out anytime via data/contributors-optout.txt on the website repo.
• You're now a Newcomer in the ladder. At 2 merged PRs you're a Regular; at 5+ you're a Fellow — that's the pool we invite first for paid research, partner-org engagements, and grant milestones. Full policy: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for the next thing to pick up? https://kcolbchain.com/invitations/ is the live list of open help-wanted / good-first-issue across every kcolbchain repo, refreshed daily.

If you want to stay in the loop on what we're shipping: https://kcolbchain.com/ROADMAP.md