fix: the swap's unknown first take shows only when the user pays that step (closes #634)

decode() now records whether the user pays the V4 step that sets the
input side, from who pays its SETTLE of its input token. That step's
Unknown (could not be decoded) line is added only then, and also when a
PERMIT2_TRANSFER_FROM of its token goes into the router, whose figure is
on its Paid to line. Paid from the router, the step takes nothing from
the wallet and never gets the line. README Transaction Decoding says so.

Model: opus-5-5

README.md

@@ -3193,13 +3193,15 @@ Currently supported:
   `Token In` after an amount that is not a quantity (a V4 open delta,
   `ALREADY_PAID`, which keep their wording on the `Amount` line, or no stated
   amount, as of a V4 exact-out step); and one whose amount the decoder does not
-  know. When the step that sets the input side states no amount and there is no
-  `Amount` line, that step gets them too whenever another take's lines give a
-  figure, or a `PERMIT2_TRANSFER_FROM` of its token gets the lines below, so
-  that figure cannot read as all that leaves. They come in step order, the step
-  that sets the input side first when it has them: `Moved from the sender`, its
-  amount, worded as the `Amount` line words one, or
-  `Unknown (could not be decoded)` when the decoder does not know it;
+  know. When the step that sets the input side states no amount, the user pays
+  it some of its input token, and there is no `Amount` line, that step gets them
+  too whenever another take's lines give a figure, or a `PERMIT2_TRANSFER_FROM`
+  of its token gives one, on the lines below or, into the router, on its
+  `Paid to` line, so that figure cannot read as all that leaves. Paid from the
+  router, that step takes nothing from the user's wallet and never gets them.
+  They come in step order, the step that sets the input side first when it has
+  them: `Moved from the sender`, its amount, worded as the `Amount` line words
+  one, or `Unknown (could not be decoded)` when the decoder does not know it;
   `Token moved`, giving the token's address as the `Token In` line does; and
   `Moved to`, which reads `The swap step that spends it`. A token the decoder
   does not know gives the `Moved from the sender` line alone. A step may also