feat(validation): occurrenceDate is required by calendar, forbidden by anchored

Both boundaries enforce both directions, because a cost's anchoring now
determines what a charge against it may carry. A slot id on an anchored cost
describes a schedule the app will not compute.

startDate granularity widens: anchored monthly costs now need a real day,
since their next due date is computed day-precisely from a charge date.

Changing anchoring on a cost that already has charges is refused. Stripping
slot ids one way is mechanical; inventing them the other way is not, and a
guess would leave real money settling nothing.

src/export/json.test.ts

@@ -563,3 +563,79 @@ describe("purchase splits validation", () => {
     expect(parseDatasetJson(exportDatasetJson(data)).purchases.length).toBe(1);
   });
 });
+
+describe("the occurrenceDate / anchoring pairing", () => {
+  function withCostAndCharge(anchoring: string, source: any): string {
+    const data: any = createSeedDataset("2026-09");
+    data.posts[0].id = "housing";
+    data.recurring = [{
+      id: "r1", name: "Bill", order: 0, archived: false,
+      amount: { amount: 300, currency: "DKK" },
+      startDate: anchoring === "lastCharge" ? "2026-09-01" : "2026-09",
+      recurrence: { kind: "everyNMonths", n: 1 },
+      anchoring, splitMode: "percent",
+      splits: [{ postId: "housing", value: 100, absorbsRemainder: true }],
+    }];
+    data.purchases = [{
+      id: "p1", date: "2026-09-20", description: "Bill",
+      total: { amount: 300, currency: "DKK" }, splitMode: "percent",
+      splits: [{ postId: "housing", value: 100, absorbsRemainder: true }],
+      schedule: null, source,
+    }];
+    return JSON.stringify(data);
+  }
+
+  test("a calendar charge REQUIRES occurrenceDate", () => {
+    expect(() => parseDatasetJson(withCostAndCharge("calendar", { recurringId: "r1" })))
+      .toThrow(/occurrence date/i);
+  });
+
+  test("a calendar charge with occurrenceDate is accepted", () => {
+    expect(() =>
+      parseDatasetJson(withCostAndCharge("calendar", { recurringId: "r1", occurrenceDate: "2026-09" })),
+    ).not.toThrow();
+  });
+
+  test("an anchored charge FORBIDS occurrenceDate", () => {
+    // Its presence describes a schedule the app will not compute.
+    expect(() =>
+      parseDatasetJson(withCostAndCharge("lastCharge", { recurringId: "r1", occurrenceDate: "2026-09" })),
+    ).toThrow(/does not settle a slot|no slots|anchored/i);
+  });
+
+  test("an anchored charge without occurrenceDate is accepted", () => {
+    expect(() => parseDatasetJson(withCostAndCharge("lastCharge", { recurringId: "r1" }))).not.toThrow();
+  });
+});
+
+describe("startDate granularity under the anchored model", () => {
+  function withCost(anchoring: string, kind: string, startDate: string): string {
+    const data: any = createSeedDataset("2026-09");
+    data.posts[0].id = "housing";
+    data.recurring = [{
+      id: "r1", name: "Bill", order: 0, archived: false,
+      amount: { amount: 300, currency: "DKK" },
+      startDate, recurrence: kind === "everyNMonths" ? { kind, n: 1 } : { kind, n: 30 },
+      anchoring, splitMode: "percent",
+      splits: [{ postId: "housing", value: 100, absorbsRemainder: true }],
+    }];
+    data.purchases = [];
+    return JSON.stringify(data);
+  }
+
+  test("anchored + everyNMonths now needs a DAY", () => {
+    expect(() => parseDatasetJson(withCost("lastCharge", "everyNMonths", "2026-09"))).toThrow(/day-granular/);
+  });
+
+  test("calendar + everyNMonths still accepts a month", () => {
+    expect(() => parseDatasetJson(withCost("calendar", "everyNMonths", "2026-09"))).not.toThrow();
+  });
+
+  test("calendar + everyNMonths still accepts a day", () => {
+    expect(() => parseDatasetJson(withCost("calendar", "everyNMonths", "2026-09-20"))).not.toThrow();
+  });
+
+  test("a calendar-impossible date is still refused", () => {
+    expect(() => parseDatasetJson(withCost("lastCharge", "everyNMonths", "2026-09-31"))).toThrow(/impossible/i);
+  });
+});