harden: state an undetermined swap input token as undetermined, not as ETH (closes #357)

tokenInfo(null) yielded "ETH (native)", so a swap whose input token the calldata never named was asserted to the user as ETH. The determination established for the output side in #353 applies unchanged: Currency is a value type over address, so native ETH arrives as the truthy zero-address string and WRAP_ETH sets it explicitly -- null can only mean undetermined.

The rule now lives in tokenInfo() itself rather than at each call site, so the redundant output-side guard added by #356 is removed; both sides read one UNNAMED_CURRENCY constant and cannot drift. Verified by execution that a genuine native-ETH input still renders as ETH, including via WRAP_ETH and a V4 zero-address PoolKey, and that the real mainnet output-side fixture is unchanged.

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)`