fix(folio): gate submit-balance property on Home route landing

totalBalance is only freshly computed when AccountCards are visible on
Home; off-Home landings return the carrier and would false-fire the
property, latching always(next(F)) to false and masking the real
double-submit bug. Skip vacuously when route is not "home".

examples/folio/sanderling/predicates.ts

@@ -39,14 +39,19 @@ export function parseTypedAmount(text: string | undefined | null): number {
 // When the last action is a tap (or double-tap) on the transaction Submit
 // button, the absolute change in total balance must equal the amount the
 // user typed. A double-submit lands two transactions and shifts the balance
-// by 2x the typed amount, tripping this check.
+// by 2x the typed amount, tripping this check. The route gate skips steps
+// whose landing screen is not Home: totalBalance is only freshly computed
+// from visible AccountCards on Home, so off-Home comparisons would read a
+// stale carrier value and false-fire.
 export function submitChangesBalanceByTypedAmount(args: {
+  route: string | null;
   lastAction: { kind?: string; on?: string | object } | null;
   typedAmount: number;
   prevTotalBalance: number;
   currTotalBalance: number;
 }): boolean {
-  const { lastAction, typedAmount, prevTotalBalance, currTotalBalance } = args;
+  const { route, lastAction, typedAmount, prevTotalBalance, currTotalBalance } = args;
+  if (route !== "home") return true;
   if (lastAction == null) return true;
   if (lastAction.kind !== "Tap" && lastAction.kind !== "DoubleTap") return true;
   const on = lastAction.on;

examples/folio/sanderling/spec.ts

@@ -98,10 +98,12 @@ const newAccountBalanceIsZero = always(
 
 // Property 2: a tap on TxnSubmit must move the total balance by exactly the
 // typed amount. A double-submit lands two transactions, so the balance shifts
-// by twice the typed amount and the check fires.
+// by twice the typed amount and the check fires. The route gate inside the
+// predicate skips off-Home landings where totalBalance.current is the carrier.
 const submitMovesBalanceByTypedAmount = always(
   next(() =>
     submitChangesBalanceByTypedAmount({
+      route: route.current,
       lastAction: lastAction.current,
       typedAmount: parseTypedAmount(txnAmountField.previous?.text),
       prevTotalBalance: totalBalance.previous ?? 0,