Explain /quote amounts and how to compare them in the API guide

[raq0x]

Why

Partners that integrate /quote directly keep putting quote.buyAmount into their generic toAmount. Ambire did this most recently (fixed in AmbireTech/ambire-common#2779). For sell orders that value is after network costs and the protocol fee. Other aggregators usually return output before gas, and the user pays gas separately. As a result, comparison UIs rank CoW last even when its price is the same.

The math is already on the How Intents Are Formed concepts page, but no other page links to it. The API guide says nothing about fees, and its quickstart spreads the whole quote response into the order.

Changes (docs/cow-protocol/integrate/api.mdx)

• New Displaying and Comparing Quotes section:
  • what sellAmount, feeAmount and buyAmount mean for a sell order
  • how to get the before-network-costs amount (buyAmount + buyAmount * feeAmount / sellAmount - partnerFee) for comparison with other aggregators, and the after-network-costs amount for after-gas ranking
  • a warning not to rank on beforeAllFees, because it also adds back the protocol fee and leaves out the partner fee
  • why executed amounts are often better than quoted
  • a short note on buy orders
  • an example (10 USDT → USDC, based on a live mainnet quote and marked as illustrative)
  • an SDK pointer to getQuoteAmountsAndCosts / beforeNetworkCosts
• Quickstart step 2 now builds the order from quoteResponse.quote. It signs sellAmount + feeAmount with feeAmount: "0", reduces buyAmount by slippage, and passes quoteId and from. This matches "Forming the Final Order" on the concepts page.
• Links to the concepts page (amount stages, partner fee, buy-order flow, final order), plus a Resources entry.

yarn build passes (onBrokenLinks / onBrokenAnchors: throw).

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