docs: name every approval-screen amount string, fix stale zero claim (closes #369)

The README said a genuine zero always renders `0.0000`, beside a
`Min. received` example that now reads `None (no minimum guaranteed)`.
It now names the two zeros stated in words: any zero `Min. received`,
and an `Amount` line showing a V4 exact-in `amountIn` of zero. That
line shows the amount of the last `PERMIT2_PERMIT` step if there is
any, otherwise of the first V2/V3 exact-in, `WRAP_ETH` or V4 swap step,
where a V4 swap step uses its first exact-in action.

A new list names every string an amount slot can show, including a
known-scale zero as `0.0000` and `Unlimited` for the router's
whole-balance value. A zero `BALANCE_CHECK_ERC20` `minBalance` is noted
as now reading the no-minimum wording.

Model: opus-5-5

TODO.md

@@ -173,6 +173,24 @@ but the review is broader than any of them.
   standalone storage entry, never part of the versioned `autistmask` profile, so
   the state schema is untouched and no existing profile is affected.
 
+- 2026-09-21: `README.md` now documents the approval screen's amount-slot
+  vocabulary and no longer contradicts itself
+  ([#369](https://git.eeqj.de/sneak/AutistMask/issues/369)). The stale claim
+  that a genuine zero always renders `0.0000` is corrected: a zero with a known
+  scale does, except where the swap screen states it in words first —
+  `Min. received` always, and the `Amount` line when the amount it shows is a V4
+  exact-in `amountIn` of zero. That line shows the amount of the last
+  `PERMIT2_PERMIT` step if there is any, otherwise of the first V2/V3 exact-in,
+  `WRAP_ETH` or V4 swap step, where a V4 swap step uses its first exact-in
+  action; any other known-scale zero there reads `0.0000`. The amount-display
+  section now names every string a slot can show and when — a formatted quantity
+  (a known-scale zero included), `Unlimited` (on a swap also the router's
+  whole-balance value), `All available (V4 open delta)`,
+  `None (no minimum guaranteed)`, base units with decimals unknown, and
+  `Unknown (not named in the calldata)` — and records that a zero `minBalance`
+  on a `BALANCE_CHECK_ERC20` step now reads `None (no minimum guaranteed)` where
+  it once read `0.0000`. Docs only.
+
 - 2026-08-30: An address no longer wraps, or is shortened to fit, in any of the
   common views ([#380](https://git.eeqj.de/sneak/AutistMask/issues/380)). The
   wallet list was the reported case: the address shared one row with the