Add boardDateRedirect guard utility for Online-Board date-window per TZ 4.1.2-R11

Shared single-purpose helper: returns redirect path when a parsed yyyymmdd
date falls outside the [-1, +14] day window, null otherwise. Six unit tests
cover both bounds, the in-window case, and locale propagation.

src/routes/[lang]/onlineboard/_guards.test.ts

@@ -0,0 +1,42 @@
+/**
+ * Unit tests for boardDateRedirect guard utility.
+ *
+ * Clock frozen at 2026-05-15 (noon local). Board window: 2026-05-14 … 2026-05-29.
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { boardDateRedirect } from "./_guards.js";
+
+describe("boardDateRedirect (clock frozen 2026-05-15)", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2026, 4, 15, 12)); // May 15 2026 noon
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("returns null for a date inside the window (today)", () => {
+    expect(boardDateRedirect("ru-ru", "20260515")).toBeNull();
+  });
+
+  it("returns null for the earliest allowed date (today - 1)", () => {
+    expect(boardDateRedirect("ru-ru", "20260514")).toBeNull();
+  });
+
+  it("returns null for the latest allowed date (today + 14)", () => {
+    expect(boardDateRedirect("ru-ru", "20260529")).toBeNull();
+  });
+
+  it("redirects when date is too early (today - 2)", () => {
+    expect(boardDateRedirect("ru-ru", "20260513")).toBe("/ru-ru/onlineboard");
+  });
+
+  it("redirects when date is too late (today + 15)", () => {
+    expect(boardDateRedirect("ru-ru", "20260530")).toBe("/ru-ru/onlineboard");
+  });
+
+  it("uses the supplied locale in the redirect path", () => {
+    expect(boardDateRedirect("en-us", "20260101")).toBe("/en-us/onlineboard");
+  });
+});

src/routes/[lang]/onlineboard/_guards.ts

@@ -0,0 +1,19 @@
+/**
+ * Shared date-window guard for Online-Board routes per TZ §4.1.2 ¶3-4.
+ *
+ * Out-of-window dates redirect to the Online-Board start page instead of 404.
+ * Parse failures (malformed URLs) continue to produce 404 via existing logic.
+ */
+
+import { isInBoardWindow } from "@/shared/dateWindow.js";
+
+/**
+ * Returns the redirect target path when the given yyyymmdd date falls outside
+ * the Online-Board [-1, +14] day window. Returns null to allow normal render.
+ *
+ * Only called after a successful URL parse — malformed dates never reach here.
+ */
+export function boardDateRedirect(locale: string, yyyymmdd: string): string | null {
+  if (!isInBoardWindow(yyyymmdd)) return `/${locale}/onlineboard`;
+  return null;
+}