Retry settlement on conflicting_nonce with backoff

[whoabuddy]

Summary

Concurrent x402 payment settlements hit nonce conflicts and fail without retry. During the Mar 24 competition livestream, 2 settlements failed at 17:00:06 UTC within the same second on different endpoints.

Current behavior

When two settlements land concurrently and hit nonce contention, both fail with conflicting_nonce:

{
  "path": "/storage/paste",
  "errorReason": "conflicting_nonce",
  "transaction": "",
  "network": "stacks:1"
}
{
  "path": "/storage/kv",
  "errorReason": "conflicting_nonce",
  "transaction": "",
  "network": "stacks:1"
}

A third request at 17:00:04 succeeded, showing the happy path works — it's purely a concurrency issue.

Expected behavior

On conflicting_nonce, retry settlement with backoff (similar to how x402-sponsor-relay handles nonce contention on the relay side).

Evidence from logs

• Mar 24 17:00: 2 concurrent failures, 1 success (same second)
• Mar 24 13:02: 1 unexpected_settle_error
• Mar 23 17:01: 1 unexpected_settle_error
• Mar 12: 7 conflicting_nonce failures in a burst (pre-fix era)

Priority

Medium — competition traffic will increase concurrent settlement pressure.

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