fix: WETH paid to the user before an UNWRAP_WETH reads as WETH (closes #607)

An UNWRAP_WETH made Token Out ETH whenever the output side was WETH,
though the router unwraps only the WETH it holds. decode() now records
where the step that set the output side paid its output, and an
UNWRAP_WETH makes the output ETH only when that step paid WETH to the
router and the router still holds WETH. When that step paid only part
of its WETH to the router, or made a payout the decoder cannot read,
the output reads ETH with no minimum, since the calldata states none
for the ETH. A BALANCE_CHECK_ERC20 pays nobody, so it is never read as
unwrapped. README Display Consistency says the same.

Model: opus-5-5

tests/uniswap.test.js

@@ -875,7 +875,7 @@ describe("uniswap decoder", () => {
         const data = buildExecute(
             solidityPacked(["uint8", "uint8"], [0x08, 0x0c]),
             [
-                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                encodeV2SwapExactIn(ROUTER_ADDR, 1000000n, 500000000000000n, [
                     USDC_ADDR,
                     SEPOLIA_WETH_ADDR,
                 ]),
@@ -958,7 +958,7 @@ describe("uniswap decoder", () => {
         const data = buildExecute(
             solidityPacked(["uint8", "uint8"], [0x08, 0x0c]),
             [
-                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                encodeV2SwapExactIn(ROUTER_ADDR, 1000000n, 500000000000000n, [
                     USDT_ADDR,
                     WETH_ADDR,
                 ]),