fix: with no Amount line, the user's takes the total adds up get lines of their own (closes #659)

When a step this decoder cannot read, an unreadable permit, or a take
of Token In with an unknown amount held back the Amount line, the
amount the step that sets the input side states, and each later take
of Token In added to it, appeared on no line, while other takes kept
their own Moved from the sender lines. Those takes now get the same
lines, first. The first step's amount gets them only when the user
pays that step (a V2 or V3 step with payerIsUser true, a V4 step the
user pays some of its input token, or a WRAP_ETH): one paid from the
router takes nothing from the wallet. Routes with an Amount line read
as before.

Model: opus-5-5

tests/uniswap.test.js

@@ -1912,9 +1912,7 @@ describe("uniswap decoder", () => {
                         ],
                     ),
                 ],
-                // Nothing read says the user pays this step, so its 400 USDT
+                first: USDT_400_GROUP,
-                // gets no lines of its own.
-                first: [],
             },
             {
                 kind: "a V3 step of 600 USDT, then a V4 step whose input cannot be read",