test: add V4 swap ERC20→ERC20 decoder regression test

Adds a test that constructs a Uniswap V4 USDT→USDC swap using
SETTLE/SWAP_EXACT_IN_SINGLE/TAKE sub-actions inside a V4_SWAP command.
Without decodeV4Swap(), the output token would be unresolvable and the
swap name would not show 'USDT → USDC'. This test fails on the old code
and passes with the decodeV4Swap() fix.

Refs: #59

tests/uniswap.test.js

@@ -1,9 +1,10 @@
-const { AbiCoder, Interface, solidityPacked } = require("ethers");
+const { AbiCoder, Interface, solidityPacked, getBytes } = require("ethers");
 const uniswap = require("../src/shared/uniswap");
 
 const ROUTER_ADDR = "0x66a9893cc07d91d95644aedd05d03f95e1dba8af";
 const USDT_ADDR = "0xdAC17F958D2ee523a2206206994597C13D831ec7";
 const WETH_ADDR = "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2";
+const USDC_ADDR = "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48";
 const USER_ADDR = "0x66133E8ea0f5D1d612D2502a968757D1048c214a";
 
 // AutistMask's first-ever swap, 2026-02-27.
@@ -256,6 +257,87 @@ describe("uniswap decoder", () => {
         expect(amount.value).toBe("5.0000 USDT");
     });
 
+    // This test validates the decodeV4Swap() fix: a V4 ERC20→ERC20 swap
+    // (USDT→USDC) where the token addresses are ONLY discoverable inside
+    // the V4_SWAP sub-actions (SETTLE/TAKE). Before decodeV4Swap() was added,
+    // command 0x10 was opaque and this would decode as "Uniswap Swap" with
+    // no token info (or "ETH → ETH"). Now it correctly shows "USDT → USDC".
+    test("decodes V4_SWAP ERC20→ERC20 tokens via SETTLE/TAKE (regression: #59)", () => {
+        // Build a V4_SWAP input with SETTLE(USDT) + SWAP_EXACT_IN_SINGLE + TAKE(USDC)
+        const V4_SETTLE = 0x0b;
+        const V4_SWAP_EXACT_IN_SINGLE = 0x06;
+        const V4_TAKE = 0x0e;
+
+        // actions: SETTLE, SWAP_EXACT_IN_SINGLE, TAKE
+        const actions = new Uint8Array([
+            V4_SETTLE,
+            V4_SWAP_EXACT_IN_SINGLE,
+            V4_TAKE,
+        ]);
+
+        // SETTLE params: (address currency, uint256 maxAmount, bool payerIsUser)
+        const settleParam = coder.encode(
+            ["address", "uint256", "bool"],
+            [USDT_ADDR, 5000000n, true],
+        );
+
+        // SWAP_EXACT_IN_SINGLE params:
+        // (tuple(address,address,uint24,int24,address) poolKey, bool zeroForOne, uint128 amountIn, uint128 amountOutMin, bytes hookData)
+        const swapParam = coder.encode(
+            [
+                "tuple(tuple(address,address,uint24,int24,address),bool,uint128,uint128,bytes)",
+            ],
+            [
+                [
+                    [
+                        USDT_ADDR,
+                        USDC_ADDR,
+                        100, // fee
+                        1, // tickSpacing
+                        "0x0000000000000000000000000000000000000000", // hooks
+                    ],
+                    true, // zeroForOne
+                    5000000n, // amountIn (5 USDT)
+                    4900000n, // amountOutMin (4.9 USDC)
+                    "0x", // hookData
+                ],
+            ],
+        );
+
+        // TAKE params: (address currency, address recipient, uint256 amount)
+        const takeParam = coder.encode(
+            ["address", "address", "uint256"],
+            [USDC_ADDR, USER_ADDR, 0n],
+        );
+
+        // Encode the V4_SWAP input: (bytes actions, bytes[] params)
+        const v4Input = coder.encode(
+            ["bytes", "bytes[]"],
+            [actions, [settleParam, swapParam, takeParam]],
+        );
+
+        // Build execute() with PERMIT2_PERMIT (0x0a) + V4_SWAP (0x10)
+        // The permit provides the input token, but V4_SWAP must provide
+        // the OUTPUT token — without decodeV4Swap, output would be unknown.
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x0a, 0x10]),
+            [encodePermit2(USDT_ADDR, 5000000n, ROUTER_ADDR), v4Input],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+        // Before decodeV4Swap fix: name would be "Swap USDT → ETH" or "Uniswap Swap"
+        // After fix: correctly identifies both tokens from V4 sub-actions
+        expect(result.name).toBe("Swap USDT \u2192 USDC");
+
+        const tokenIn = result.details.find((d) => d.label === "Token In");
+        expect(tokenIn.value).toContain("USDT");
+
+        const steps = result.details.find((d) => d.label === "Steps");
+        expect(steps.value).toContain("V4 Swap");
+    });
+
     test("handles unknown tokens gracefully", () => {
         const fakeToken = "0x1111111111111111111111111111111111111111";
         const data = buildExecute(