fix: a V4 step of chained swap actions shows what the user receives (closes #563)

A V4 step's input side is now the currency its SETTLE or SETTLE_ALL
names, or else the one currency that some swap action spends and none
buys; its output side is the currency TAKE or TAKE_ALL names, or else
the one that some swap action buys and none spends. Amount and Min.
received come from the actions that spend the input and buy the output,
summed when several exact-in actions each state one. A step with an
unread swap action is named only by SETTLE, SETTLE_ALL, TAKE and
TAKE_ALL, and shows no figure. Before, the first swap action set both
sides, SETTLE_ALL and TAKE_ALL were not read, and a SWAP_EXACT_OUT was
read as spending its path's last currency.

Model: opus-5-5

README.md

@@ -1171,8 +1171,8 @@ read:
   amount of one step that names an input token or amount: the last
   `PERMIT2_PERMIT` step if there is one, otherwise the first V2 or V3 exact-in,
   V2 exact-out, `WRAP_ETH` or V4 swap step. A V2 exact-out step gives its
-  `amountInMax`, and a V4 swap step the `amountIn` of its first readable
-  exact-in action.
+  `amountInMax`, and a V4 swap step the `amountIn` of the swap action that
+  spends its input currency, as described below.
 - `Whatever an earlier step sent to the pair (V2 already paid)`: the swap's
   `Amount` line, when the amount it shows is a V2 exact-in `amountIn` of zero.
   The router reads that zero as "the pair already holds the input tokens": the
@@ -1193,17 +1193,35 @@ read:
 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
 that side, or `Unknown (could not be decoded)` instead when a V4 swap action in
-the transaction could not be read (see Transaction Decoding below). A V4 step
-whose swap action could not be read still sets each side as a swap step does,
-and a side its `SETTLE` or `TAKE` did not name is then unknown: no other step's
-token or figure is shown for it. An `UNWRAP_WETH` step makes `Token Out` ETH
-only when the output side is WETH, on mainnet or Sepolia, or when no step set
-the output side; a WETH `Min. received` figure then reads in ETH. Otherwise
-`Token Out` and `Min. received` keep the output side's own token and figure,
-whether the swap was paid in ETH or in a token: a V2 exact-out swap that buys
-USDC and then unwraps the WETH it did not spend shows USDC. The token permission
-warning on the signature screen has its own amount wording, including `Unknown`;
-the SignApproval section below describes it.
+the transaction could not be read (see Transaction Decoding below). A V4 step's
+sides follow three rules:
+
+1. Its input side is the currency its `SETTLE` or `SETTLE_ALL` names, or else
+   the one currency that some swap action in the step spends and none buys. Its
+   output side is the currency its `TAKE` or `TAKE_ALL` names, or else the one
+   currency that some swap action buys and none spends. If there is not exactly
+   one such currency, that side is unknown. A step that buys WETH with USDT and
+   USDC with that WETH, with nothing naming its sides, reads `Swap USDT → USDC`.
+2. `Amount` is the `amountIn` of the swap action that spends the input currency.
+   If several exact-in actions spend it, each with a stated amount, it is their
+   sum; in any other case there is no `Amount` line. The decoder reads no amount
+   from an exact-out action, and an open delta is not a stated amount.
+   `Min. received` is found the same way, from the `amountOutMin` of the actions
+   that buy the output currency.
+3. If any swap action in the step could not be read, only `SETTLE`,
+   `SETTLE_ALL`, `TAKE` and `TAKE_ALL` name its sides, and there is no `Amount`
+   or `Min. received` line. Such a step still sets each side as a swap step
+   does, and no other step's token or figure is shown for a side they did not
+   name.
+
+An `UNWRAP_WETH` step makes `Token Out` ETH only when the output side is WETH,
+on mainnet or Sepolia, or when no step set the output side; a WETH
+`Min. received` figure then reads in ETH. Otherwise `Token Out` and
+`Min. received` keep the output side's own token and figure, whether the swap
+was paid in ETH or in a token: a V2 exact-out swap that buys USDC and then
+unwraps the WETH it did not spend shows USDC. The token permission warning on
+the signature screen has its own amount wording, including `Unknown`; the
+SignApproval section below describes it.
 
 The swap's `Deadline` line is the router's deadline as a UTC date and time, e.g.
 `2026-02-27 08:25:51`. A JavaScript date reaches only to 275760-09-13 00:00:00