fix: a WRAP_ETH says where its ETH goes (closes #747)

A WRAP_ETH into the router now always reads `Moved to: The router`. That is
true whichever step then takes the WETH. Other takes keep
`The swap step that spends it`.

A later WRAP_ETH of CONTRACT_BALANCE to anyone but the router now gets a
`Moved from the sender` line reading `All the ETH the transaction carries`,
and `Moved to` names the recipient. It gets none after an UNWRAP_WETH into
the router, or while the router holds ETH from a V4 step.

README states both rules.

Model: opus-5-5

src/shared/uniswap.js

@@ -404,6 +404,11 @@ const OPEN_DELTA = Symbol("v4-open-delta");
 // that nothing is swapped.
 const ALREADY_PAID = Symbol("v2-already-paid");
 
+// A later WRAP_ETH of CONTRACT_BALANCE wraps all the ETH the router holds, and
+// so all the ETH the transaction carries. The calldata states no quantity for
+// it. The approval screen's Value line gives that ETH.
+const TRANSACTION_ETH = Symbol("transaction-eth");
+
 // The router's whole-balance amount, universal-router
 // `ActionConstants.CONTRACT_BALANCE`: a V2 exact-in given it as `amountIn`
 // pays the pair all the router holds of the input token.
@@ -416,6 +421,7 @@ const CONTRACT_BALANCE = 2n ** 255n;
 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 TRANSACTION_ETH_AMOUNT = "All the ETH the transaction carries";
 const NO_MINIMUM = "None (no minimum guaranteed)";
 
 // Permit2 amounts are uint160; the maximum is Permit2's "unbounded".
@@ -1267,9 +1273,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // ERC-20 token from the router takes none of that token from the
         // sender.
         let inputPaidByUser = false;
-        // The recipient of a WRAP_ETH that set the input side, as written,
-        // when it is not the router; otherwise null. That WRAP_ETH pays its
-        // WETH there, not to a swap step (see the Moved to lines).
+        // The recipient of a WRAP_ETH that set the input side, as written;
+        // otherwise null. That WRAP_ETH pays its WETH there (see the Moved to
+        // lines).
         let wrapEthRecipient = null;
         // The last PERMIT2_PERMIT this decoder read before a step set the
         // input side, or null (see permitStands after the loop).
@@ -1289,6 +1295,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         let minOutput = null;
         let outputEstablished = false;
         let hasUnwrapWeth = false;
+        // Set once an UNWRAP_WETH pays ETH to the router, which a later
+        // WRAP_ETH of CONTRACT_BALANCE then wraps.
+        let unwrappedIntoRouter = false;
         // Set when an UNWRAP_WETH pays out, as ETH, the output side's WETH,
         // which the router holds; cleared whenever the output side is set
         // again, or a later step may pay that ETH out as WETH (see after the
@@ -1368,9 +1377,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // After the loop each joins the Amount line or gets lines of its own
         // (see permitStands). A take with `atMost` set may take less than
         // `amount`: it never joins the Amount line, and its line reads
-        // "Up to" and the amount. A take with a `recipient`, the ETH of a
-        // later WRAP_ETH that pays anyone but the router, never joins the
-        // Amount line either: its Moved to line names that recipient.
+        // "Up to" and the amount. A take with a `recipient` is the ETH of a
+        // later WRAP_ETH. If that recipient is not the router, the take never
+        // joins the Amount line either: its Moved to line names that recipient.
         const userPaid = [];
         // A swap step the user pays for (`payerIsUser` true) takes `amount`
         // of `token` from the sender, or at most that when `atMost` is set. A
@@ -1906,9 +1915,7 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                                 w.amount,
                             );
                             inputPaidByUser = true;
-                            if (!isRouter(w.recipient)) {
-                                wrapEthRecipient = w.recipient;
-                            }
+                            wrapEthRecipient = w.recipient;
                         } else if (
                             w.amount > 0n &&
                             w.amount !== CONTRACT_BALANCE
@@ -1916,17 +1923,33 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                             // The ETH of a stated amount comes from the
                             // transaction's value. ETH an earlier step left in
                             // the router may pay part of it, and the sender
-                            // then pays at most that amount. CONTRACT_BALANCE
-                            // states no amount to take.
+                            // then pays at most that amount.
                             userPaid.push({
                                 token: "0x0000000000000000000000000000000000000000",
                                 amount: w.amount,
                                 atMost: heldByRouter.has(
                                     "0x0000000000000000000000000000000000000000",
                                 ),
-                                recipient: isRouter(w.recipient)
-                                    ? null
-                                    : w.recipient,
+                                recipient: w.recipient,
+                            });
+                        } else if (
+                            w.amount === CONTRACT_BALANCE &&
+                            !isRouter(w.recipient) &&
+                            !unwrappedIntoRouter &&
+                            !heldByRouter.has(
+                                "0x0000000000000000000000000000000000000000",
+                            )
+                        ) {
+                            // It pays the recipient all the ETH the transaction
+                            // carries. It gets no lines after an UNWRAP_WETH
+                            // into the router, or while the router holds ETH
+                            // from a V4 step: Uniswap's routes pay back what
+                            // their swap did not spend this way, and carry no
+                            // ETH.
+                            userPaid.push({
+                                token: "0x0000000000000000000000000000000000000000",
+                                amount: TRANSACTION_ETH,
+                                recipient: w.recipient,
                             });
                         }
                         addPayee(w.recipient);
@@ -2169,6 +2192,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                             heldByRouter.has(w),
                         );
                         WETH_ADDRESSES.forEach((w) => heldByRouter.delete(w));
+                        if (isRouter(u.recipient)) {
+                            unwrappedIntoRouter = true;
+                        }
                         if (isRouter(u.recipient) && heldWeth) {
                             heldByRouter.set(
                                 "0x0000000000000000000000000000000000000000",
@@ -2266,8 +2292,8 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // What the steps take that the Amount line does not give, as
         // { token, amount, atMost, recipient }. A null token is a step this
         // decoder did not read (see spentUnread()), or a token it does not
-        // know. `atMost` is as in `userPaid`. Only the take of a WRAP_ETH that
-        // pays elsewhere has a recipient (see wrapEthRecipient and userPaid).
+        // know. `atMost` is as in `userPaid`. Only the take of a WRAP_ETH has
+        // a recipient (see wrapEthRecipient and userPaid).
         const spentFromSender = [];
         let total = null;
         // The takes from the sender that `total` adds up, each a quantity, in
@@ -2315,7 +2341,7 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                 typeof total === "bigint" &&
                 typeof t.amount === "bigint" &&
                 !t.atMost &&
-                !present(t.recipient)
+                (!present(t.recipient) || isRouter(t.recipient))
             ) {
                 total += t.amount;
                 inTotal.push(t);
@@ -2413,13 +2439,14 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         }
 
         // The text of an amount the swap steps spend, for the Amount line and
-        // the lines for other tokens below it. 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. `atMost` marks an amount that is only the most a
-        // take may be (see userPaid).
+        // the lines for other tokens below it. Four amounts need no scale to
+        // describe and are named rather than formatted: V4's open delta, V2's
+        // already-paid zero and the ETH the transaction carries, none of which
+        // is a quantity at all (see OPEN_DELTA, ALREADY_PAID and
+        // TRANSACTION_ETH), and an unbounded permit. Those three tests come
+        // first — the sentinels are not bigints and cannot be compared with
+        // one. `atMost` marks an amount that is only the most a take may be
+        // (see userPaid).
         const spentText = (amount, info, atMost) => {
             if (amount === OPEN_DELTA) {
                 return { raw: OPEN_DELTA_AMOUNT, display: OPEN_DELTA_AMOUNT };
@@ -2430,6 +2457,12 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                     display: ALREADY_PAID_AMOUNT,
                 };
             }
+            if (amount === TRANSACTION_ETH) {
+                return {
+                    raw: TRANSACTION_ETH_AMOUNT,
+                    display: TRANSACTION_ETH_AMOUNT,
+                };
+            }
             if (amount >= MAX_UINT160) {
                 return { raw: "Unlimited", display: "Unlimited" };
             }
@@ -2489,8 +2522,8 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // of the lines below for a PERMIT2_TRANSFER_FROM. An amount this
         // decoder does not know is said so, and a step it did not read gets
         // that one line alone. A take with no recipient goes to the swap step
-        // that spends it. A take with a recipient goes to that recipient;
-        // `MSG_SENDER` means the sender.
+        // that spends it. A take with a recipient goes to that recipient:
+        // the router, the sender as `MSG_SENDER`, or an address.
         for (const s of spentFromSender) {
             details.push({
                 label: "Moved from the sender",
@@ -2514,6 +2547,8 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                         label: "Moved to",
                         value: "The swap step that spends it",
                     });
+                } else if (isRouter(s.recipient)) {
+                    details.push({ label: "Moved to", value: "The router" });
                 } else if (s.recipient.toLowerCase() === MSG_SENDER) {
                     details.push({ label: "Moved to", value: "The sender" });
                 } else {