fix: a zero-minimum balance check never changes the output side (closes #649)

A BALANCE_CHECK_ERC20 of the sender with a zero minimum guarantees
nothing. It replaced the output side whenever that side held no
minimum, so after an UNWRAP_WETH that drops the minimum, a zero check
of USDC named USDC as Token Out. Such a check now leaves alone an
output side that an earlier step set. It still sets the output side
when no earlier step did. A check with a positive minimum is unchanged.

Model: opus-5-5

README.md

@@ -1283,9 +1283,8 @@ see Transaction Decoding), do not always read as a number. They can read:
   [#359](https://git.eeqj.de/sneak/AutistMask/issues/359), a zero `minBalance`
   read `0.0000` when the token's scale was known. The router passes a balance
   check whenever the balance is at least `minBalance`, so a zero `minBalance`
-  guarantees nothing: it sets `Token Out` and `Min. received` only when the
-  output side holds no minimum, not even a zero one, at the point the check is
-  reached, and otherwise leaves the current token and figure in place. A nonzero
+  guarantees nothing. A check with a zero `minBalance` sets `Token Out` and
+  `Min. received` only when no earlier step set the output side. A nonzero
   `minBalance` sets both lines, as a swap step does. Only a check of the
   transaction's sender's own balance sets either line; a check of any other
   address sets neither (see Transaction Decoding). A `BALANCE_CHECK_ERC20` step
@@ -3397,12 +3396,13 @@ Currently supported:
   `Balance Check (of another address)`. A check whose input does not decode,
   such as one whose owner word has a nonzero top byte (the router reads the
   owner from the word's last 20 bytes), likewise sets neither, and it is listed
-  as `Balance Check (could not be decoded)`. When a swap step, `TAKE` or
-  `TAKE_PORTION` pays the router any token, the swap's output or another, a
-  `WRAP_ETH` pays it WETH, or an `UNWRAP_WETH` into the router turns WETH a step
-  paid it into ETH, that token stays in the router unless a later step pays out
-  or spends all the router holds of it: a `SWEEP` of that token to anyone but
-  the router, an `UNWRAP_WETH` when it is WETH, a `WRAP_ETH` of
+  as `Balance Check (could not be decoded)`. A check with a zero `minBalance`
+  never changes an output side that an earlier step set. When a swap step,
+  `TAKE` or `TAKE_PORTION` pays the router any token, the swap's output or
+  another, a `WRAP_ETH` pays it WETH, or an `UNWRAP_WETH` into the router turns
+  WETH a step paid it into ETH, that token stays in the router unless a later
+  step pays out or spends all the router holds of it: a `SWEEP` of that token to
+  anyone but the router, an `UNWRAP_WETH` when it is WETH, a `WRAP_ETH` of
   `CONTRACT_BALANCE` when it is ETH, a `TRANSFER` of `CONTRACT_BALANCE`, a
   `PAY_PORTION` of 10,000 bips or a `PAY_PORTION_FULL_PRECISION` of 1e18 of that
   token to anyone but the router, an `ACROSS_V4_DEPOSIT_V3` of

src/shared/uniswap.js

@@ -1629,13 +1629,14 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                     } else {
                         // 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. A check the router lets fail guarantees
-                        // nothing either, so it is read as a zero one. Any
-                        // other minBalance sets the output side as a swap
-                        // does. It pays nobody.
+                        // nothing. A check with a zero minBalance leaves
+                        // alone an output side that an earlier step set,
+                        // token included. A check the router lets fail
+                        // guarantees nothing either, so it is read as a zero
+                        // one. Any other minBalance sets the output side as a
+                        // swap does. The check pays nobody.
                         const minBalance = allowedToFail ? 0n : b.minBalance;
-                        if (!(minBalance === 0n && present(minOutput))) {
+                        if (!(minBalance === 0n && outputEstablished)) {
                             setOutput(i, b.token, minBalance, []);
                         }
                         keepsRouterWeth = true;

tests/uniswapRecipient.test.js

@@ -688,6 +688,34 @@ describe("an UNWRAP_WETH makes the output ETH only when the router holds it", ()
         ]);
     });
 
+    // The same route, then a balance check of the sender's USDC with a zero
+    // minimum. The check guarantees nothing, so Token Out stays ETH
+    // (https://git.eeqj.de/sneak/AutistMask/issues/649).
+    test("the same route, then a zero balance check of USDC, still reads as ETH with no minimum", () => {
+        const result = decode(
+            execute(
+                v4Swap(
+                    SWAP_USDT_FOR_WETH,
+                    SETTLE_USDT,
+                    take(WETH, MSG_SENDER, (ONE_WETH * 6n) / 10n),
+                    take(WETH, ADDRESS_THIS),
+                ),
+                unwrapWeth(MSG_SENDER),
+                balanceCheck(USDC, 0n),
+            ),
+        );
+        expect(result.name).toBe("Swap USDT → ETH");
+        expect(detail(result, "Token Out").value).toBe("ETH");
+        expect(labels(result)).toEqual([
+            "Protocol",
+            "Token In",
+            "Amount",
+            "Token Out",
+            "Steps",
+            "Deadline",
+        ]);
+    });
+
     // A TAKE the decoder cannot read may pay part of the WETH to the user.
     test("one V4 step paying WETH to the router beside a TAKE that cannot be read, then an UNWRAP_WETH, reads as ETH with no minimum", () => {
         const result = decode(