harden: gate swap amounts on presence, not truthiness, so a figure always matches its token (closes #359)

Token and amount were gated on truthiness and gated independently. An address is never falsy once set but 0n is, so a hop supplying a zero amount fixed the token permanently while leaving the amount open, and the next hop's figure was rendered against the first hop's token at that token's scale -- 0.5 WETH shown as 500000000000.0000 USDT. Nine defective gates are now presence checks, and fourteen address gates were converted defensively so a sixth instance of this class cannot grow.

Zero is not one thing. Established from v4-periphery: OPEN_DELTA is 0 and V4Router substitutes the full credit unconditionally, so a zero V4 exact-in amount means "swap the whole balance" and now reads "All available (V4 open delta)" rather than 0.0000, which would assert the exact inverse. amountOutMinimum gets no such mapping and universal-router special-cases only CONTRACT_BALANCE, so a zero minimum and a zero V2/V3 amount stay literal -- a zero floor reads "None (no minimum guaranteed)".

closes #364

TODO.md

@@ -45,6 +45,21 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: A swap amount and the token it is counted in now always come from
+  the same hop, on both sides of the approval screen
+  ([#359](https://git.eeqj.de/sneak/AutistMask/issues/359) and
+  [#364](https://git.eeqj.de/sneak/AutistMask/issues/364), the output and input
+  halves of one gate, fixed as one unit). `src/shared/uniswap.js` gated the
+  token and the amount on truthiness and independently; an address is never
+  falsy once set but an amount of `0n` is, so a hop supplying a zero amount
+  fixed the token and left the amount open, and the next hop's figure was then
+  rendered against the first hop's token at that token's scale — 0.5 WETH shown
+  as `500000000000.0000 USDT`, and an earlier hop's `Min. received` shown for a
+  final leg that guarantees nothing. Both sides are now set as a pair through
+  explicit presence, a zero slippage floor reads `None (no minimum guaranteed)`,
+  and V4's `OPEN_DELTA` (an `amountIn` of zero, which `V4Router` reads as "swap
+  the whole open credit") reads `All available (V4 open delta)` instead of
+  `0.0000`.
 - 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