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 says which zeros are stated in words first: any zero
`Min. received`, and a zero V4 exact-in `amountIn` only when that step
sets the `Amount` line, since a Permit2 permit sets it whenever present.

A new list names every string an amount slot can show and when,
including a known-scale zero as `0.0000` and `Unlimited` for the
router's whole-balance value. It also records that a zero `minBalance`
on a `BALANCE_CHECK_ERC20` step now reads the no-minimum wording.
Docs only.

Model: opus-5-5

README.md

@@ -882,9 +882,18 @@ On those screens, when the truncated string would contain no digit from 1 to 9
 and the value does, the amount is extended to its first significant digit
 instead: `0.000000000000000001 DAI`, not `0.0000 DAI`. The test is on the whole
 truncated string, integer part included, so `1.00005` still shows as `1.0000` —
-the exception only fires where the entire displayed figure would read as zero. A
-genuine zero still renders `0.0000`, and truncation stays truncation: `0.99999`
-shows as `0.9999`, never rounded up.
+the exception only fires where the entire displayed figure would read as zero.
+Truncation stays truncation: `0.99999` shows as `0.9999`, never rounded up. A
+genuine zero reaching this rule renders `0.0000`. The swap screen states a zero
+in words before it gets here in only two places. Its `Min. received` line reads
+`None (no minimum guaranteed)` for any zero minimum. Its `Amount` line reads
+`All available (V4 open delta)` for a V4 exact-in `amountIn` of zero, which V4
+treats as the whole open credit rather than a quantity — but only when that V4
+step is the one that sets the `Amount` line. A Permit2 permit sets it whenever
+one is among the steps, so a swap carrying a permit shows the permit's amount
+there and never the open-delta wording. Every other zero amount with a known
+scale renders `0.0000`, the swap's `Amount` line included; the list of
+amount-slot strings below says where.
 
 The rule and its exception live in `src/shared/amountDisplay.js` as
 `truncateAmount()` and `truncateAmountNeverZero()`. Everything the approval and
@@ -936,6 +945,49 @@ and compare against. Reading the stored field directly instead answers `null`
 for a bundled or tracked token the explorer merely omitted, which is not a
 refusal the wallet has any reason to make.
 
+**Every string an amount slot can show:** taken together, the exceptions above
+mean an amount line on the dApp approval screen (and the wait/success/error
+screens that carry a figure forward) shows one of a fixed set of strings, not
+always a number:
+
+- A formatted quantity, e.g. `17.1900 USDT`: the token's scale is known. A
+  figure at or above the floor is truncated to four decimals, one below it is
+  extended to its first significant digit, and a genuine zero renders `0.0000`,
+  e.g. `0.0000 USDT`. That zero reaches the ERC-20 `approve`/`transfer` line for
+  an amount of zero, and the swap's `Amount` line when the step that sets it
+  states zero: a Permit2 permit, a V2 or V3 exact-in `amountIn`, or a `WRAP_ETH`
+  (`0.0000 ETH`). This is `truncateAmountNeverZero()`
+  (`src/shared/amountDisplay.js`).
+- `Unlimited`: named rather than formatted, since it needs no scale to describe.
+  On the ERC-20 line it is an `approve` of the `uint256` maximum, an unbounded
+  allowance (`src/popup/views/approval.js`); `transfer` has no such case. On a
+  swap's `Amount` line it is any input amount at or above the `uint160` maximum,
+  whichever step set the line (`src/shared/uniswap.js`): a Permit2 permit at
+  that maximum, which is an unbounded permit, or a V2 or V3 exact-in `amountIn`
+  or `WRAP_ETH` amount that large. That includes the router's whole-balance
+  value (`2^255`), which tells the router to use all it holds of the token — a
+  quantity decided at execution, not an allowance.
+- `All available (V4 open delta)`: a V4 exact-in swap whose `amountIn` is zero.
+  V4 reads that zero as "use the whole open delta", not as a literal zero, so
+  the calldata states no quantity at all. Swap `Amount` line only, and only when
+  that V4 step is the one that sets the line. A Permit2 permit sets it instead
+  whenever one is among the steps, and so does any earlier swap or `WRAP_ETH`
+  step; the line then shows what that step stated (`src/shared/uniswap.js`).
+- `None (no minimum guaranteed)`: a zero minimum — the swap guarantees nothing
+  back. It is a literal zero slippage floor on a V2/V3/V4 swap, and it also
+  reaches a `BALANCE_CHECK_ERC20` step: a zero `minBalance`, which once rendered
+  `0.0000` beside the token symbol, now reads this. Swap `Min. received` line
+  (`src/shared/uniswap.js`).
+- `<amount> base units (decimals unknown)`: the token's scale could not be
+  resolved, so the base-unit integer is shown with that caveat rather than
+  formatted (see Unknown token scale above). Reaches both the ERC-20 amount line
+  and the swap's `Amount` and `Min. received` (`unknownDecimalsAmount()` in
+  `src/shared/approvalAmount.js`).
+- `Unknown (not named in the calldata)`: not an amount but the currency itself —
+  the `Token In` or `Token Out` line when nothing in the calldata established
+  which token, shown beside the amount and, like the strings above, a sentence
+  rather than a value (`src/shared/uniswap.js`).
+
 #### Partial USD totals
 
 Prices are fetched for the top 25 tokens only, so an address can hold assets the

TODO.md

@@ -154,6 +154,22 @@ 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 only when a V4 exact-in of zero
+  is the step that sets it. A Permit2 permit sets that line whenever one is
+  present, so a zero permit, a zero V2/V3 exact-in `amountIn` or a zero
+  `WRAP_ETH` still 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