fix: the swap decoder reads a V2 already-paid zero and a zero balance check as the router does (closes #415)

A V2 exact-in amountIn of zero is the router's ALREADY_PAID marker: an
earlier step sent the tokens to the pair and the swap spends all of them.
Amount showed 0.0000 for it; it now reads "Whatever an earlier step sent
to the pair (V2 already paid)", in the style of the V4 open delta line.

A BALANCE_CHECK_ERC20 passes whenever the balance is at least minBalance,
so a zero one guarantees nothing. It now sets the output side only when
that side holds no minimum at the point the check is reached; a nonzero
one sets the output side as before.

README's Display Consistency text and TODO.md are updated to match.

Model: opus-5-5

src/shared/uniswap.js

@@ -84,17 +84,31 @@ function present(value) {
 //
 // `amountOutMinimum` gets no such mapping: V4Router compares it directly
 // (`if (amountOut < params.amountOutMinimum) revert V4TooLittleReceived`), so
-// a zero minimum is a literal zero slippage floor and is stated as one. Nor do
-// the V2/V3 paths have it — universal-router's `V3SwapRouter.v3SwapExactInput`
+// a zero minimum is a literal zero slippage floor and is stated as one. Nor
+// does the V3 path have it — universal-router's `V3SwapRouter.v3SwapExactInput`
 // special-cases only `ActionConstants.CONTRACT_BALANCE` (1<<255), never zero —
-// so a zero `amountIn` there is a literal zero and is displayed as one.
+// so a zero V3 `amountIn` is a literal zero and is displayed as one. The V2
+// exact-in path gives zero a meaning of its own: see ALREADY_PAID.
 const OPEN_DELTA = Symbol("v4-open-delta");
 
-// The two amount lines that state a fact instead of a quantity. Same register
-// as UNNAMED_CURRENCY — a sentence in the value slot, so it cannot be misread
-// as a number — and deliberately not a third phrasing of "not named": these
-// say different things.
+// The Universal Router's V2 exact-in spells "the pair already holds the input
+// tokens" as an amount of zero: universal-router
+// `contracts/libraries/Constants.sol` declares
+// `uint256 internal constant ALREADY_PAID = 0` ("Used for identifying cases
+// when a v2 pair has already received input tokens"), and
+// `V2SwapRouter.v2SwapExactInput` makes no payment of its own when `amountIn`
+// equals it. The swap then spends whatever an earlier step sent to the pair.
+// As with OPEN_DELTA, the calldata states no quantity, and "0.0000" would say
+// that nothing is swapped.
+const ALREADY_PAID = Symbol("v2-already-paid");
+
+// The amount lines that state a fact instead of a quantity. Same register as
+// UNNAMED_CURRENCY — a sentence in the value slot, so it cannot be misread as a
+// number — and deliberately not another phrasing of "not named": these say
+// different things.
 const OPEN_DELTA_AMOUNT = "All available (V4 open delta)";
+const ALREADY_PAID_AMOUNT =
+    "Whatever an earlier step sent to the pair (V2 already paid)";
 const NO_MINIMUM = "None (no minimum guaranteed)";
 
 // Permit2 amounts are uint160; the maximum is Permit2's "unbounded".
@@ -185,6 +199,7 @@ function decodeBalanceCheck(input) {
 // Decode V2_SWAP_EXACT_IN (command 0x08) input bytes.
 // ABI: (address recipient, uint256 amountIn, uint256 amountOutMin,
 //        address[] path, bool payerIsUser)
+// A zero `amountIn` is read the way the router reads it, as ALREADY_PAID.
 function decodeV2SwapExactIn(input) {
     try {
         const d = coder.decode(
@@ -192,7 +207,7 @@ function decodeV2SwapExactIn(input) {
             input,
         );
         return {
-            amountIn: d[1],
+            amountIn: d[1] === 0n ? ALREADY_PAID : d[1],
             amountOutMin: d[2],
             tokenIn: d[3][0],
             tokenOut: d[3][d[3].length - 1],
@@ -502,7 +517,13 @@ function decode(data, toAddress, sources) {
 
                 if (cmdId === 0x0e) {
                     const b = decodeBalanceCheck(inputs[i]);
-                    if (b) setOutput(b.token, b.minBalance);
+                    // The router passes this check whenever the owner holds at
+                    // least minBalance, so a zero one guarantees nothing and
+                    // does not replace a minimum an earlier step stated. Any
+                    // other minBalance sets the output side as a swap does.
+                    if (b && !(b.minBalance === 0n && present(minOutput))) {
+                        setOutput(b.token, b.minBalance);
+                    }
                 }
 
                 if (cmdId === 0x00) {
@@ -623,14 +644,20 @@ function decode(data, toAddress, sources) {
         }
 
         if (present(inputAmount)) {
-            // Two amounts need no scale to describe and are named rather than
-            // formatted: V4's open delta, which is not a quantity at all (see
-            // OPEN_DELTA), and an unbounded permit. The open-delta test comes
-            // first — the sentinel is not a bigint and cannot be compared with
-            // one.
+            // Three amounts need no scale to describe and are named rather
+            // than formatted: V4's open delta and V2's already-paid zero,
+            // neither of which is a quantity at all (see OPEN_DELTA and
+            // ALREADY_PAID), and an unbounded permit. Those two tests come
+            // first — the sentinels are not bigints and cannot be compared
+            // with one.
             let amount;
             if (inputAmount === OPEN_DELTA) {
                 amount = { raw: OPEN_DELTA_AMOUNT, display: OPEN_DELTA_AMOUNT };
+            } else if (inputAmount === ALREADY_PAID) {
+                amount = {
+                    raw: ALREADY_PAID_AMOUNT,
+                    display: ALREADY_PAID_AMOUNT,
+                };
             } else if (inputAmount >= MAX_UINT160) {
                 amount = { raw: "Unlimited", display: "Unlimited" };
             } else if (hasV2ExactOut) {