harden: say a swap's input token is unknown rather than calling it ETH (closes #357)

`tokenInfo(null)` answered `{symbol: "ETH", decimals: 18}`, so a swap whose
calldata never named an input currency rendered `Token In: ETH (native)` and
titled itself `Swap ETH -> X`. The approval screen asserted the user was paying
native ETH when nothing had established it.

Null is not how native ETH arrives. v4-core declares `type Currency is address`
and wraps `address(0)` for it; a user-defined value type over `address` carries
the plain `address` ABI encoding, so a native-ETH currency reaches every decode
site here as the truthy string
`0x0000000000000000000000000000000000000000`, and WRAP_ETH sets that same
explicit zero address. A null token means undetermined, on the input side as on
the output side.

The collapse is removed from `tokenInfo()` itself rather than guarded at each
call site, so both sides of the screen answer the same condition the same way:
null refuses, the zero address is still ETH at 18 decimals. `Token In` gains
the refusal branch `Unknown (not named in the calldata)`, now a shared constant
with the `Token Out` branch it must match.

TODO.md

@@ -45,6 +45,16 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: A swap whose input token the calldata never named is said to be
+  unknown instead of being called ETH
+  ([#357](https://git.eeqj.de/sneak/AutistMask/issues/357)), the twin on the
+  input side of [#353](https://git.eeqj.de/sneak/AutistMask/issues/353). A null
+  `inputToken` rendered as `Token In: ETH (native)` and titled the swap
+  `Swap ETH -> X`, asserting the user was paying native ETH when nothing in the
+  calldata said so. The null-means-ETH collapse is now gone from `tokenInfo()`
+  itself rather than guarded at each call site: null is refused, and native ETH
+  keeps arriving as the explicit zero address that `WRAP_ETH` and V4's
+  `Currency.wrap(address(0))` both use.
 - 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)`