fix(store): an explicit source: undefined clears a purchase's tag before validation

updatePurchase resolved the pair it validates with `??`, which cannot tell
"no source key" from "source: undefined, clear the association". Untagging an
anchored charge while dropping it to a month-only date was refused, citing the
cost being removed. Resolve by key presence instead.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/store/actions.test.ts

@@ -1157,6 +1157,51 @@ describe("a charge for an anchored cost needs a day-granular date", () => {
     expect(() => occurrencesByMonth(d, "2026-12")).not.toThrow();
   });
 
+  /**
+   * `updatePurchase` resolves the pair it validates, and `??` cannot tell "no
+   * source key in this update" from "source: undefined, clear the
+   * association" — which is exactly what `PurchaseDialog.save()` sends when
+   * the owner picks "None", deliberately, because `Object.assign` would
+   * otherwise leave the stored association untouched.
+   *
+   * Pre-wave that conflation was harmless: the stored source was always
+   * self-consistent, so re-validating it changed nothing. Adding the date to
+   * the check is what made it bite — the store began refusing a write on the
+   * strength of a pair that will not exist once the write lands, citing a cost
+   * the owner is in the middle of removing. The flow is the obvious one: a
+   * tagged purchase, "use the month only", read the dialog's line saying the
+   * date must be exact, decide to drop the tag instead, and Save is refused.
+   */
+  describe("an explicit source: undefined CLEARS the association, and is validated as such", () => {
+    test("untagging and dropping to a month-only date in one write is accepted", () => {
+      const { d, post, cost } = seeded({ kind: "everyNMonths", n: 1 });
+      const purchase = actions.addPurchase(d, charge(post.id, cost.id, "2026-09-20"));
+      expect(() =>
+        actions.updatePurchase(d, purchase.id, { source: undefined, date: "2026-09" }),
+      ).not.toThrow();
+      expect(d.purchases[0]!.source).toBeUndefined();
+      expect(d.purchases[0]!.date).toBe("2026-09");
+      expect(() => occurrencesByMonth(d, "2026-12")).not.toThrow();
+    });
+
+    test("untagging alone is still accepted", () => {
+      const { d, post, cost } = seeded({ kind: "everyNMonths", n: 1 });
+      const purchase = actions.addPurchase(d, charge(post.id, cost.id, "2026-09-20"));
+      expect(() => actions.updatePurchase(d, purchase.id, { source: undefined })).not.toThrow();
+      expect(d.purchases[0]!.source).toBeUndefined();
+    });
+
+    test("an update that omits `source` entirely still re-validates the STORED one", () => {
+      // The other half of the same distinction: omitting the key must NOT be
+      // read as clearing the tag, or an unrelated edit could carry a stored
+      // anchored charge to a month-only date unchecked.
+      const { d, post, cost } = seeded({ kind: "everyNMonths", n: 1 });
+      const purchase = actions.addPurchase(d, charge(post.id, cost.id, "2026-09-20"));
+      expect(() => actions.updatePurchase(d, purchase.id, { date: "2026-09" })).toThrow(/Phone/);
+      expect(d.purchases[0]!.source).toEqual({ recurringId: cost.id });
+    });
+  });
+
   test("a month-only charge date on a CALENDAR cost is still fine — it settles a slot, it is not an anchor", () => {
     const d = createSeedDataset("2026-09");
     const post = d.posts[0]!;

src/store/actions.ts

@@ -234,12 +234,15 @@ export function updatePurchase(
 ): void {
   const purchase = requirePurchase(draft, purchaseId);
   const resolved: Partial<Omit<Purchase, "id">> = { ...changes };
-  // The RESOLVED pair, same `??` style `updateRecurringCost` uses for
-  // anchoring/recurrence above: the incoming `source` if this update sets
-  // one, else the one already stored — so touching an unrelated field still
+  // The RESOLVED pair: the incoming `source` if this update carries the key,
+  // else the one already stored — so touching an unrelated field still
   // re-validates whatever source is already there, and a `source` this
-  // update DOES set is checked against the cost it now names.
-  requireSourcePairing(draft, changes.source ?? purchase.source, changes.date ?? purchase.date);
+  // update DOES set is checked against the cost it now names. Key presence,
+  // not `??`: `source: undefined` CLEARS the association (what
+  // `PurchaseDialog` sends for "None"), and `??` would validate the stored
+  // tag the write is removing.
+  const source = "source" in changes ? changes.source : purchase.source;
+  requireSourcePairing(draft, source, changes.date ?? purchase.date);
   if (changes.total) {
     resolved.total = roundMoneyValue(draft, changes.total);
   }