feat(inbox): gasless inbox sends via relay sponsorPayment

[biwasxyz]

Gasless inbox sends.

• X402_SPONSOR_FEE_PAYER (optional env var): when set, the inbox 402 advertises extra.feePayer: <address>. x402-stacks clients (>=2.1.0) then sign a sponsored sBTC transfer with fee 0. When unset, the 402 is unchanged and senders pay their own gas.
• Sponsored payments are settled with env.X402_RELAY.sponsorPayment(txHex, settle) (feat: sponsor x402 payments over the service binding only, sign and broadcast in one step x402-sponsor-relay#436). The relay sponsors and broadcasts, or refuses with a code:
  • INSUFFICIENT_FUNDS → 402
  • RATE_LIMITED → 429 (new)
  • SENDER_NONCE_CONFLICT → 409
  • BROADCAST_FAILED → 502, counted toward the circuit breaker
  • SPONSOR_EXHAUSTED → RELAY_ERROR; it was mapped to the sender's INSUFFICIENT_FUNDS
• The old submitPayment queue path and the HTTP /relay fallback are no longer used for inbox payments. A sponsored payment with no binding is refused.
• Self-paid payments still go through the relay's /settle as before.

Deploy after the relay PR. Set X402_SPONSOR_FEE_PAYER to the relay's sponsor address once the relay is live; unset it to turn gasless sends off.

Inbox tests: 446 passing.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	landing-page	5368e6c	Commit Preview URL Branch Preview URL	Oct 02 2026, 04:32 AM