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

TODO.md

@@ -45,6 +45,16 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The swap decoder reads two router zeros the way the router does
+  ([#415](https://git.eeqj.de/sneak/AutistMask/issues/415)). A V2 exact-in
+  `amountIn` of zero means an earlier step already sent the tokens to the pair;
+  `Amount` showed `0.0000` for it and now reads
+  `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 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
   showed only the decoded text, with bidirectional and zero-width characters