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

tests/uniswap.test.js

@@ -831,6 +831,104 @@ describe("uniswap decoder", () => {
         expect(detail(result, "Min. received").value).toBe("0.9900 USDC");
     });
 
+    // https://git.eeqj.de/sneak/AutistMask/issues/415 — the router's V2
+    // exact-in reads an amountIn of zero as universal-router
+    // Constants.ALREADY_PAID: an earlier step sent the tokens to the pair, and
+    // the swap uses all of them. Against 375998b this read "0.0000 USDT".
+    test("a V2 exact-in already-paid amountIn is named, not printed as zero", () => {
+        const data = buildExecute(
+            "0x08",
+            [
+                encodeV2SwapExactIn(
+                    USER_ADDR,
+                    0n, // Constants.ALREADY_PAID
+                    500000000000000n,
+                    [USDT_ADDR, WETH_ADDR],
+                ),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token In").value).toContain("USDT");
+        expect(detail(result, "Amount").value).toBe(
+            "Whatever an earlier step sent to the pair (V2 already paid)",
+        );
+        expect(detail(result, "Amount").rawValue).toBe(
+            "Whatever an earlier step sent to the pair (V2 already paid)",
+        );
+        expect(detail(result, "Min. received").value).toBe("0.0005 WETH");
+    });
+
+    // https://git.eeqj.de/sneak/AutistMask/issues/415 — the router passes a
+    // BALANCE_CHECK_ERC20 whenever the balance is at least minBalance, so a
+    // zero one guarantees nothing. Against 375998b it replaced the swap's
+    // output side: Token Out = USDC, Min. received = "None (no minimum
+    // guaranteed)".
+    test("a zero balance check keeps the minimum a swap step stated", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x08, 0x0e]),
+            [
+                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                    USDT_ADDR,
+                    WETH_ADDR,
+                ]),
+                encodeBalanceCheck(USER_ADDR, USDC_ADDR, 0n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("WETH");
+        expect(detail(result, "Min. received").value).toBe("0.0005 WETH");
+    });
+
+    // A nonzero balance check still replaces the output side, as before.
+    test("a nonzero balance check replaces the minimum a swap step stated", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x08, 0x0e]),
+            [
+                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                    USDT_ADDR,
+                    WETH_ADDR,
+                ]),
+                encodeBalanceCheck(USER_ADDR, USDC_ADDR, 2000000n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("USDC");
+        expect(detail(result, "Min. received").value).toBe("2.0000 USDC");
+    });
+
+    // With no minimum stated before it, a zero balance check is what sets the
+    // output side, and it guarantees nothing.
+    test("a zero balance check with no earlier minimum states no minimum", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x0b, 0x0e]),
+            [
+                encodeWrapEth(ROUTER_ADDR, 1000000000000000000n),
+                encodeBalanceCheck(USER_ADDR, USDT_ADDR, 0n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("USDT");
+        expect(detail(result, "Min. received").value).toBe(
+            "None (no minimum guaranteed)",
+        );
+    });
+
     // Pins what https://git.eeqj.de/sneak/AutistMask/pulls/356 changed without
     // testing: a non-swap execute() carrying only PERMIT2_PERMIT names no
     // output currency, so it says so and titles itself "Uniswap Swap" rather