fix(folio): an amount over the app's cap spends no window budget

the corpus reaches TxnSubmit with 999999999999999999999, AMOUNT_REGEX takes it
and AddTransactionViewModel refuses it against MAX_TRANSACTION_AMOUNT_CENTS, so
counting it was budget a double submit could hide behind.

examples/folio/sanderling/predicates.ts

@@ -283,6 +283,9 @@ export function countSubmitsInWindow(args: {
   return { reported, next: fresh ? 0 : reported };
 }
 
+// Folio's own cap, in cents (core/data/Repository.kt).
+const MAX_TRANSACTION_AMOUNT_CENTS = 100_000_000;
+
 // Could the app have committed anything for that submit? The amount field as
 // the LANDING frame shows it is the form state the tap read: the tap changes
 // nothing about it, and one action runs per step, so nothing else could have.
@@ -302,14 +305,19 @@ export function countSubmitsInWindow(args: {
 // nothing to say. Measured over four recorded android runs, 19, 11, 25 and 25
 // of 35, 26, 42 and 42 submit taps landed with the amount field empty.
 //
-// An amount too large for a Kotlin Long is refused by the app too, and still
-// counts here: over-counting can only cost a detection, and the reading that
-// would have to prove the overflow is a float that cannot hold the number.
+// An amount over Folio's cap is refused before any coroutine starts
+// (MAX_TRANSACTION_AMOUNT_CENTS, checked in both AddTransactionViewModel.submit
+// and Repository.createTransaction), and the fuzzer's corpus reaches the button
+// with one: "999999999999999999999" passes AMOUNT_REGEX, so the field takes it.
+// Float is precise enough to say which side of the cap an amount is on. The cap
+// is 1e8, every integer cent up to 2^53 is exact, and an amount far enough above
+// it to be inexact is far enough above it to be refused.
 export function submitCouldCommit(amountText: string | undefined): boolean {
   if (amountText === undefined) return true;
   const trimmed = amountText.trim().replace(/,/g, "");
   if (!/^\d+(\.\d{1,2})?$/.test(trimmed)) return false;
-  return /[1-9]/.test(trimmed);
+  if (!/[1-9]/.test(trimmed)) return false;
+  return Number(trimmed) * 100 <= MAX_TRANSACTION_AMOUNT_CENTS;
 }
 
 // Parses formatCents output like "$5.00", "-$1,234.56", "+$0.50" back to

pkg/spec/test/folio-submit-window.test.ts

@@ -89,11 +89,32 @@ test("a submit the app must have refused does not spend the window's budget", ()
   }
 });
 
+// Folio caps a transaction at $1,000,000.00 (MAX_TRANSACTION_AMOUNT_CENTS, in
+// core/data/Repository.kt), and AddTransactionViewModel.submit refuses anything
+// over it before a coroutine starts. The fuzzer's corpus carries
+// "999999999999999999999", AMOUNT_REGEX lets it into the field and it reaches
+// the button, so this is a refusal the window used to pay for.
+test("an amount over the app's cap cannot commit", () => {
+  for (const amountText of ["1000000.01", "1,000,001", "999999999999999999999"]) {
+    assert.deepEqual(
+      countSubmitsInWindow({
+        previousCount: 0,
+        lastAction: { kind: "Tap", on: submitOn },
+        amountText,
+        fresh: false,
+      }),
+      { reported: 0, next: 0 },
+      `amount ${JSON.stringify(amountText)} was counted as a possible commit`,
+    );
+  }
+});
+
 // The field as the landing frame shows it, which is the form state the tap read:
 // nothing between the two changes it. Anywhere but the transaction screen there
-// is no field to read, and unknown has to count.
+// is no field to read, and unknown has to count. The cap itself is an amount the
+// app takes, so it counts too.
 test("an amount that could commit, or that nobody could read, spends the budget", () => {
-  for (const amountText of ["5", "0.01", "1,000", "999999999999999999999", undefined]) {
+  for (const amountText of ["5", "0.01", "1,000", "1000000.00", "999999.99", undefined]) {
     assert.deepEqual(
       countSubmitsInWindow({
         previousCount: 0,