feat(domain): add MonthId arithmetic

Uses a month ordinal rather than Date, so month stepping is timezone-proof.

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

src/domain/months.test.ts

@@ -0,0 +1,68 @@
+// src/domain/months.test.ts
+import { test, expect, describe } from "bun:test";
+import {
+  monthOf,
+  nextMonth,
+  prevMonth,
+  compareMonths,
+  monthRange,
+  monthsOfYear,
+  addMonths,
+  yearOf,
+} from "./months.ts";
+
+test("monthOf extracts the month from an ISO date", () => {
+  expect(monthOf("2026-09-14")).toBe("2026-09");
+});
+
+describe("month stepping", () => {
+  test("nextMonth crosses the year boundary", () => {
+    expect(nextMonth("2026-11")).toBe("2026-12");
+    expect(nextMonth("2026-12")).toBe("2027-01");
+  });
+
+  test("prevMonth crosses the year boundary", () => {
+    expect(prevMonth("2026-01")).toBe("2025-12");
+  });
+
+  test("addMonths handles multi-year spans in both directions", () => {
+    expect(addMonths("2026-09", 6)).toBe("2027-03");
+    expect(addMonths("2026-09", -12)).toBe("2025-09");
+    expect(addMonths("2026-09", 0)).toBe("2026-09");
+  });
+});
+
+test("compareMonths orders chronologically", () => {
+  expect(compareMonths("2026-01", "2026-02")).toBeLessThan(0);
+  expect(compareMonths("2027-01", "2026-12")).toBeGreaterThan(0);
+  expect(compareMonths("2026-05", "2026-05")).toBe(0);
+});
+
+describe("monthRange", () => {
+  test("is inclusive of both ends", () => {
+    expect(monthRange("2026-11", "2027-01")).toEqual([
+      "2026-11",
+      "2026-12",
+      "2027-01",
+    ]);
+  });
+
+  test("returns a single month when both ends match", () => {
+    expect(monthRange("2026-03", "2026-03")).toEqual(["2026-03"]);
+  });
+
+  test("returns empty when `to` precedes `from`", () => {
+    expect(monthRange("2026-05", "2026-04")).toEqual([]);
+  });
+});
+
+test("monthsOfYear returns twelve padded months", () => {
+  const months = monthsOfYear(2026);
+  expect(months).toHaveLength(12);
+  expect(months[0]).toBe("2026-01");
+  expect(months[11]).toBe("2026-12");
+});
+
+test("yearOf parses the year", () => {
+  expect(yearOf("2026-09")).toBe(2026);
+});