test(spec): cover balanceMatchesAddedSum single, sum-match, over, under cases

Pins the sum-based predicate: a single new row matching delta and two new
rows summing to delta both hold; two-row over-sum (double-submit) and
under-sum cases both violate.

pkg/spec/test/folio-balance-predicate.test.ts

@@ -0,0 +1,42 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import {
+  balanceMatchesAddedSum,
+  type LedgerRow,
+} from "../../../examples/folio/sanderling/predicates.ts";
+
+function row(key: string, signed: number): LedgerRow {
+  return { key, signed };
+}
+
+test("single new row whose signed amount matches the balance delta holds", () => {
+  const previous: LedgerRow[] = [row("a", 100)];
+  const current: LedgerRow[] = [row("a", 100), row("b", 500)];
+  assert.equal(balanceMatchesAddedSum(previous, current, 100, 600), true);
+});
+
+test("two new rows whose sum matches the balance delta holds", () => {
+  const previous: LedgerRow[] = [];
+  const current: LedgerRow[] = [row("a", 500), row("b", 300)];
+  assert.equal(balanceMatchesAddedSum(previous, current, 0, 800), true);
+});
+
+test("double-submit case: two identical rows with sum exceeding the delta violates", () => {
+  const previous: LedgerRow[] = [];
+  const current: LedgerRow[] = [row("a", 500), row("b", 500)];
+  assert.equal(balanceMatchesAddedSum(previous, current, 0, 500), false);
+});
+
+test("partial-credit case: two new rows whose sum falls short of the delta violates", () => {
+  const previous: LedgerRow[] = [];
+  const current: LedgerRow[] = [row("a", 200), row("b", 100)];
+  assert.equal(balanceMatchesAddedSum(previous, current, 0, 500), false);
+});
+
+test("no new rows holds vacuously regardless of balance delta", () => {
+  const previous: LedgerRow[] = [row("a", 100)];
+  const current: LedgerRow[] = [row("a", 100)];
+  assert.equal(balanceMatchesAddedSum(previous, current, 100, 100), true);
+  assert.equal(balanceMatchesAddedSum(previous, current, 100, 999), true);
+});