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

A V4 step's input side is the currency SETTLE or SETTLE_ALL names, else
the one currency some swap action spends and none buys; its output side
is the currency TAKE or TAKE_ALL names, else the one some swap action
buys and none spends. Amount and Min. received come from those actions,
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. A side a step with a swap action leaves
unknown shows no other step's token or figure. Before, the first swap
action set both sides, SETTLE_ALL and TAKE_ALL went unread, 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
@@ -1194,16 +1194,37 @@ 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.
+with a swap action sets both sides as a swap step does, following 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, and no other step's token or figure
+   is shown for it. 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. A side they did not name is unknown, and no other
+   step's token or figure is shown for it.
+
+A V4 step with no swap action sets only the sides its `SETTLE`, `SETTLE_ALL`,
+`TAKE` or `TAKE_ALL` names.
+
+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