Sandbox mode

Add sandbox: true to /quote (or /x402/quote). The price is still live and real. The flag rides inside the quoteToken, so every later call sees it with nothing else to pass. Sandbox swaps are never metered or paid.

Call	In sandbox
`/execute`	Opens nothing at any venue. Every answer carries `"sandbox": true`
`/submit-signature`	Doesn’t forward the signature. Returns a stand-in `statusRef`
`/submit-transaction`	Runs its checks, sends nothing. Returns a stand-in `txHash`
`/status`	Answers `success` at once, with the quoted output as `deliveredAmount`. Any `ref` works

What /execute returns. Venues that would open something to build the swap (an order, a deposit address, a channel) return a stand-in DEPOSIT instead: send to it and the funds go to your user’s own wallet (refundAddress, else userAddress). These are NEAR Intents, Rift, Garden, Layerswap, Houdini Swap, Hinkal, Chainflip, RAVN’s two-hop routes and THORChain from Bitcoin. To test TRANSACTION handling, quote another venue, for example with onlyVenues.

Every other venue returns its real transaction or typed data, built live, and Relay from Bitcoin returns the real deposit address it opened when it quoted. Sent or signed to the venue, it swaps for real. Check "sandbox": true and never send it. The SDK’s executeAndTrack sends nothing on a sandbox execution.

On x402, the USDC charge for the call still applies in sandbox mode.