feat(store): add setRuleFrom and removeRuleFrom

setRuleFrom replaces rather than appends for a month that already has a
version, keeping the effective rule unambiguous, and keeps the series sorted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/store/actions.test.ts

@@ -302,3 +302,79 @@ describe("settings", () => {
     expect(data.fxRates).toEqual([]);
   });
 });
+
+describe("rule versions", () => {
+  const pct = (percent: number) => ({ kind: "percentOfIncome" as const, percent });
+
+  test("setRuleFrom adds a version to a post that had none", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-04", pct(10));
+    expect(data.posts[0]!.rules).toEqual([{ from: "2026-04", rule: pct(10) }]);
+  });
+
+  test("setRuleFrom REPLACES an existing version for the same month", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-04", pct(10));
+    actions.setRuleFrom(data, postId, "2026-04", pct(15));
+    expect(data.posts[0]!.rules).toHaveLength(1);
+    expect(data.posts[0]!.rules[0]!.rule).toEqual(pct(15));
+  });
+
+  test("versions are kept sorted regardless of insertion order", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-07", pct(15));
+    actions.setRuleFrom(data, postId, "2026-04", pct(10));
+    actions.setRuleFrom(data, postId, "2026-05", pct(12));
+    expect(data.posts[0]!.rules.map((v) => v.from)).toEqual([
+      "2026-04",
+      "2026-05",
+      "2026-07",
+    ]);
+  });
+
+  test("a fixed rule is rounded at the mutation boundary", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-04", {
+      kind: "fixed",
+      amount: { amount: 12.345, currency: "DKK" },
+    });
+    const stored = data.posts[0]!.rules[0]!.rule;
+    expect(stored.kind === "fixed" && stored.amount.amount).toBe(12.35);
+  });
+
+  test("a percentage above 100 is stored, not clamped", () => {
+    const data = draft();
+    actions.setRuleFrom(data, data.posts[0]!.id, "2026-04", pct(150));
+    expect(data.posts[0]!.rules[0]!.rule).toEqual(pct(150));
+  });
+
+  test("removeRuleFrom deletes exactly that version", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-04", pct(10));
+    actions.setRuleFrom(data, postId, "2026-07", pct(15));
+    actions.removeRuleFrom(data, postId, "2026-04");
+    expect(data.posts[0]!.rules).toEqual([{ from: "2026-07", rule: pct(15) }]);
+  });
+
+  test("removing a month that has no version is a no-op, not an error", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleFrom(data, postId, "2026-04", pct(10));
+    actions.removeRuleFrom(data, postId, "2026-09");
+    expect(data.posts[0]!.rules).toHaveLength(1);
+  });
+
+  test("both throw on an unknown post id", () => {
+    expect(() => actions.setRuleFrom(draft(), "ghost", "2026-04", pct(10))).toThrow(
+      /Unknown post: ghost/,
+    );
+    expect(() => actions.removeRuleFrom(draft(), "ghost", "2026-04")).toThrow(
+      /Unknown post: ghost/,
+    );
+  });
+});

src/store/actions.ts

@@ -1,5 +1,5 @@
 import { newId } from "../domain/seed.ts";
-import { monthOf } from "../domain/months.ts";
+import { compareMonths, monthOf } from "../domain/months.ts";
 import { roundMoney } from "../domain/money.ts";
 import type {
   Currency,
@@ -13,6 +13,7 @@ import type {
   Purchase,
   PurchaseId,
   Rule,
+  RuleVersion,
   Schedule,
   Split,
 } from "../domain/types.ts";
@@ -144,6 +145,35 @@ export function movePost(draft: Dataset, postId: PostId, direction: -1 | 1): voi
   [a.order, b.order] = [b.order, a.order];
 }
 
+/**
+ * Inserts or replaces the version starting at `from`, keeping `rules` sorted.
+ * Replacing rather than appending is what keeps the effective rule
+ * unambiguous: two versions sharing a month would both be "in effect", and
+ * ruleAt would resolve to whichever the sort happened to leave last.
+ */
+export function setRuleFrom(
+  draft: Dataset,
+  postId: PostId,
+  from: MonthId,
+  rule: Rule,
+): void {
+  const post = requirePost(draft, postId);
+  const version: RuleVersion = { from, rule: roundRule(rule) };
+  const existing = post.rules.findIndex((v) => v.from === from);
+  if (existing === -1) post.rules.push(version);
+  else post.rules[existing] = version;
+  post.rules.sort((a, b) => compareMonths(a.from, b.from));
+}
+
+/**
+ * Deletes the version starting at `from`. Removing the earliest one means the
+ * months before the next version now resolve to no rule, and allocate zero.
+ */
+export function removeRuleFrom(draft: Dataset, postId: PostId, from: MonthId): void {
+  const post = requirePost(draft, postId);
+  post.rules = post.rules.filter((v) => v.from !== from);
+}
+
 export function addPurchase(
   draft: Dataset,
   purchase: Omit<Purchase, "id">,