fix(swap): report relay's real minimumamount as minimumoutput (false zero-slippage signal)

[gomesalexandre]

what — Relay reports a false zero-slippage floor

RelayProvider.GetQuote (sdk/swap/relay.go) set:

MinimumOutput: expectedOutput, // Relay is solver-based: no AMM slippage

That comment is false. A live POST api.relay.link/quote for ETH->USDC returns isFixedRate: false with a genuine gap between currencyOut.amount and currencyOut.minimumAmount:

amount        = 1758580426
minimumAmount = 1740994621   → ~1.000% gap

Copying expectedOutput into MinimumOutput hands every caller a permanently false zero-slippage floor. Any pre-sign card, slippage-risk warning, or downstream check that reads Quote.MinimumOutput shows "no downside" for a swap that can actually settle ~1% lower than expected.

how

Add minimumAmount to the relayCurrencyOut response struct and use it as MinimumOutput, falling back to expectedOutput only when the API omits the field. Regression test (httptest with distinct amount/minimumAmount) asserts MinimumOutput tracks minimumAmount and falls back safely when absent.

risk

Low, strictly corrective — MinimumOutput now reflects the API's real floor instead of an optimistic copy. go build ./sdk/swap/ + go vet clean; test passes under the go.mod toolchain (GOTOOLCHAIN=go1.24.2 — the package test binary doesn't link on Go >=1.24 due to a pinned bytedance/sonic/go:linkname issue, unrelated to this change).

receipts

Field + gap live-verified against the production Relay API (curl above). Found by a fund-safety audit of the recipes swap providers (the same audit that surfaced the MayaChain zero-slippage CRITICAL, recipes#597). Related lower-severity findings from that audit (Relay ignoring an explicit AffiliateBps: 0 opt-out; 1inch/Jupiter/LiFi/Uniswap not forwarding req.ToleranceBps) are being triaged separately.

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