Keep seeded PRNG output strictly below 1 in checkout-pricing test

TODO.md

@@ -509,15 +509,3 @@ Fix direction: in `applyArg`, when `VALUE_FLAGS[arg]` exists but `next` is
 `undefined`, raise a clear "missing value for <flag>" usage error rather than
 pushing the flag to `positional`; keep consuming/returning true when a value is
 present.
-
-### 5. Seeded PRNG `rand()` can return exactly 1
-
-`test/lib/checkout-pricing-consistency.test.ts` — `makeRng`'s `rand` returns
-`seed / 0x7fffffff`, which is `1` when `seed === 0x7fffffff`. That lets `pick`
-index one past the array and `randInt` return `hi + 1`. Astronomically unlikely
-for the fixed seeds in use (and identical to `main`), but incorrect.
-
-Fix direction: normalize with `seed / 0x80000000` (or clamp below 1). Note this
-changes the seeded sequence, so re-baseline any hardcoded expectations and
-confirm the property test still passes; best landed on its own so the corpus
-shift is reviewed deliberately.

test/lib/checkout-pricing-consistency.test.ts

@@ -118,7 +118,10 @@ const makeRng = (initialSeed: number): Rng => {
   let seed = initialSeed;
   const rand = () => {
     seed = (seed * 1103515245 + 12345) & 0x7fffffff;
-    return seed / 0x7fffffff;
+    // Divide by 2^31 (not 2^31-1) so the result is always in [0, 1): at
+    // seed === 0x7fffffff the old divisor returned exactly 1, letting `pick`
+    // index past the array and `randInt` return hi + 1.
+    return seed / 0x80000000;
   };
   const pick = <T>(xs: readonly T[]): T => xs[Math.floor(rand() * xs.length)]!;
   const randInt = (lo: number, hi: number) =>

test/shared/payment-signature.test.ts

@@ -58,7 +58,8 @@ const excludedKeys: [string, Record<string, string>][] = [
 /** Tiny deterministic PRNG so the fuzz loop is repeatable. */
 const lcg = (seed: number) => () => {
   seed = (seed * 1103515245 + 12345) & 0x7fffffff;
-  return seed / 0x7fffffff;
+  // Divide by 2^31 (not 2^31-1) so the result stays in [0, 1).
+  return seed / 0x80000000;
 };
 
 describeWithEnv("payment price signature", { encryptionKey: true }, () => {