feat(export): add JSON export and validated import

Import validates far enough that the fold cannot throw on imported data:
exactly one remainder-absorbing split per purchase, at least one split,
every split.postId resolving to a real post, every currency supported,
and every MonthId well-formed and in range (01-12). Over-budget and
percentages not summing to 100 remain permitted.

Tightens MONTH_ID to /^\d{4}-(0[1-9]|1[0-2])$/ (shape-only regex would
let "2026-13" through import and silently misfile inside the balance
fold, mirroring the defect already fixed in src/domain/months.ts).

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

src/export/json.test.ts

@@ -0,0 +1,118 @@
+import { test, expect, describe } from "bun:test";
+import {
+  exportDatasetJson,
+  parseDatasetJson,
+  describeDataset,
+  ImportValidationError,
+  exportFilename,
+} from "./json.ts";
+import { createSeedDataset } from "../domain/seed.ts";
+import { monthView, yearView } from "../domain/views.ts";
+import type { Dataset } from "../domain/types.ts";
+
+function populated(): Dataset {
+  const data = createSeedDataset("2026-01");
+  data.months[0]!.income = { amount: 20000, currency: "DKK" };
+  data.purchases.push({
+    id: "p1",
+    date: "2026-01-15",
+    description: "Console",
+    total: { amount: 3000, currency: "DKK" },
+    splitMode: "percent",
+    splits: [
+      { postId: data.posts[0]!.id, value: 70, absorbsRemainder: true },
+      { postId: data.posts[2]!.id, value: 30, absorbsRemainder: false },
+    ],
+    schedule: {
+      slices: [
+        { month: "2026-01", amount: { amount: 1000, currency: "DKK" } },
+        { month: "2026-02", amount: { amount: 2000, currency: "DKK" } },
+      ],
+    },
+  });
+  return data;
+}
+
+describe("round trip", () => {
+  test("export then import yields an equal dataset", () => {
+    const data = populated();
+    expect(parseDatasetJson(exportDatasetJson(data))).toEqual(data);
+  });
+
+  test("derived output is identical after a round trip", () => {
+    const data = populated();
+    const restored = parseDatasetJson(exportDatasetJson(data));
+    expect(monthView(restored, "2026-01")).toEqual(monthView(data, "2026-01"));
+    expect(yearView(restored, 2026)).toEqual(yearView(data, 2026));
+  });
+
+  test("export is human-readable, indented JSON", () => {
+    expect(exportDatasetJson(createSeedDataset("2026-01"))).toContain("\n  ");
+  });
+});
+
+describe("validation", () => {
+  test("rejects malformed JSON", () => {
+    expect(() => parseDatasetJson("{not json")).toThrow(ImportValidationError);
+  });
+
+  test("rejects a dataset missing a required collection", () => {
+    const data = populated() as any;
+    delete data.purchases;
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/purchases/);
+  });
+
+  test("rejects a purchase whose splits reference an unknown post", () => {
+    const data = populated();
+    data.purchases[0]!.splits[0]!.postId = "ghost";
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/ghost/);
+  });
+
+  test("rejects a purchase with no remainder-absorbing split", () => {
+    const data = populated();
+    data.purchases[0]!.splits[0]!.absorbsRemainder = false;
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/absorbsRemainder/);
+  });
+
+  test("rejects a purchase with two remainder-absorbing splits", () => {
+    const data = populated();
+    data.purchases[0]!.splits[1]!.absorbsRemainder = true;
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/exactly one/i);
+  });
+
+  test("rejects a purchase with no splits", () => {
+    const data = populated();
+    data.purchases[0]!.splits = [];
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/at least one split/i);
+  });
+
+  test("rejects an unsupported currency", () => {
+    const data = populated() as any;
+    data.settings.baseCurrency = "GBP";
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/GBP/);
+  });
+
+  test("rejects a malformed MonthId", () => {
+    const data = populated();
+    data.months[0]!.id = "2026-1";
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/2026-1/);
+  });
+
+  test("rejects a MonthId with an out-of-range month", () => {
+    const data = populated();
+    data.months[0]!.id = "2026-13";
+    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/2026-13/);
+  });
+});
+
+test("describeDataset counts what an import would replace", () => {
+  expect(describeDataset(populated())).toEqual({
+    posts: 3,
+    months: 1,
+    purchases: 1,
+  });
+});
+
+test("exportFilename is stable and sortable", () => {
+  expect(exportFilename("2026-09")).toBe("budget-2026-09.json");
+});