harden: say a swap's output token is unknown rather than calling it ETH (closes #353)

`tokenInfo(null)` answers `{symbol: "ETH", decimals: 18}`, and the V4 arm of
`decode()` could take a step's `amountOutMin` while leaving `outputToken`
null, because the token assignment was gated on `if (v4.tokenOut)`. The
approval screen then named ETH as the output and formatted `Min. received` at
18 decimals for a swap whose output currency the calldata never stated.

Null is not how V4 spells native ETH. v4-core declares `type Currency is
address` and wraps `address(0)` for native ETH, and a `Currency` is
ABI-encoded as a plain address word, so every decode site here gets back the
truthy string `0x0000000000000000000000000000000000000000` for it — which
`tokenInfo()` already names ETH — and an UNWRAP_WETH output is caught
separately. Nothing that genuinely outputs ETH arrives as null; only a step
whose output currency did not decode does.

So a null output token is now treated as undetermined: `Token Out` reads
`Unknown (not named in the calldata)` and `Min. received` falls to the
base-unit refusal from
#340 instead of being scaled at 18.
A V4 step that supersedes an earlier step's `Min. received` without naming a
currency also clears the earlier step's token, so that figure is never read
against the token a previous hop named.

TODO.md

@@ -45,6 +45,18 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: A swap whose output token the calldata never named is said to be
+  unknown instead of being called ETH
+  ([#353](https://git.eeqj.de/sneak/AutistMask/issues/353)). `tokenInfo(null)`
+  answers `{symbol: "ETH", decimals: 18}`, and a V4 step could take the
+  `Min. received` figure while naming no output currency, so the approval screen
+  stated the wrong asset at the wrong scale. Null is not how V4 spells native
+  ETH: v4-core's `type Currency is address` wraps `address(0)` for it, which
+  reaches the decoder as the truthy string
+  `0x0000000000000000000000000000000000000000` and is named ETH there already.
+  `Token Out` now reads `Unknown (not named in the calldata)` and
+  `Min. received` falls to the base-unit refusal from
+  [#340](https://git.eeqj.de/sneak/AutistMask/issues/340).
 - 2026-08-23: The background no longer reads or writes the shared `state`
   singleton ([#324](https://git.eeqj.de/sneak/AutistMask/issues/324)), which
   also closes the cold-worker wrong-chain send