fix(folio): extract balanceMatchesAddedSum predicate as testable helper

Move the ledger-balance-vs-added-rows predicate into a pure helper module
so the property's logic is unit-testable in isolation. Marks the sanderling
example as an ES module so cross-package ESM imports resolve under node.

examples/folio/sanderling/package.json

@@ -1,6 +1,7 @@
 {
   "name": "folio",
   "private": true,
+  "type": "module",
   "description": "Minimal sanderling integration example.",
   "devDependencies": {
     "@sanderling/spec": "file:../../../pkg/spec"

examples/folio/sanderling/predicates.ts

@@ -0,0 +1,22 @@
+export interface LedgerRow {
+  key: string;
+  signed: number;
+}
+
+// When new ledger rows appear, the balance delta must equal the SUM of the
+// new rows' signed amounts. Single-row sums and multi-row sums BOTH must
+// match the delta; a double-submit landing two rows whose total is twice
+// the balance change trips the sum check.
+export function balanceMatchesAddedSum(
+  previousRows: readonly LedgerRow[],
+  currentRows: readonly LedgerRow[],
+  previousBalance: number,
+  currentBalance: number,
+): boolean {
+  const previousKeys = new Set(previousRows.map(row => row.key));
+  const added = currentRows.filter(row => !previousKeys.has(row.key));
+  if (added.length === 0) return true;
+  const delta = currentBalance - previousBalance;
+  const addedSum = added.reduce((sum, row) => sum + row.signed, 0);
+  return addedSum === delta;
+}