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

README.md

@@ -1006,9 +1006,10 @@ read:
   [#359](https://git.eeqj.de/sneak/AutistMask/issues/359), a zero `minBalance`
   read `0.0000` when the token's scale was known. The router passes a balance
   check whenever the balance is at least `minBalance`, so a zero `minBalance`
-  guarantees nothing: it sets `Token Out` and `Min. received` only when no
-  earlier step stated a minimum, and otherwise leaves that step's token and
-  figure in place. A nonzero `minBalance` sets both lines, as a swap step does.
+  guarantees nothing: it sets `Token Out` and `Min. received` only when the
+  output side holds no minimum, not even a zero one, at the point the check is
+  reached, and otherwise leaves the current token and figure in place. A nonzero
+  `minBalance` sets both lines, as a swap step does.
 
 The swap's `Token In` and `Token Out` lines name a currency, not an amount; each
 reads `Unknown (not named in the calldata)` when the decoder found no token for

TODO.md

@@ -52,7 +52,8 @@ but the review is broader than any of them.
   `Whatever an earlier step sent to the pair (V2 already paid)`. A
   `BALANCE_CHECK_ERC20` with a zero `minBalance` guarantees nothing, yet it
   replaced the minimum an earlier swap step stated, so `Min. received` read
-  `None (no minimum guaranteed)`; it now leaves that minimum in place. A nonzero
+  `None (no minimum guaranteed)`; it now sets the output side only when that
+  side holds no minimum at the point the check is reached. A nonzero
   `minBalance` still sets the output side.
 - 2026-10-04: The signature screen shows a personal message as the bytes that
   are signed ([#403](https://git.eeqj.de/sneak/AutistMask/issues/403)). It