desloppify: code health overhaul plus abuse hardening (140 green, strict 87.2)

Honest baseline after removing 45 gaming suppressions. Structural: lib/game plus root hooks extraction, CORS origin echo, DEV_ORIGINS layering. Fixes: error-cause binding, discord 5xx split, null-cache guard, co-located tests, rejecting-fetch coverage, naming and typing cleanup, socket controller, cache bound, cascade helpers, latch resets. Hardening: per-IP rate limit on API routes, deterministic-miss UX, retry cooldown plus cap. Suite 140 green, build plus lint clean.

app/not-found.test.tsx

@@ -0,0 +1,16 @@
+import { describe, expect, it, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import NotFound from "@/app/not-found";
+
+vi.mock("@/lib/i18n/get-locale", () => ({
+    getLocale: async () => "en" as const,
+}));
+
+describe("NotFound page", () => {
+    it("renders the 404 block with a home link", async () => {
+        render(await NotFound());
+        expect(screen.getByText("404")).toBeDefined();
+        const home = screen.getByRole("link", { name: /home/i });
+        expect(home.getAttribute("href")).toBe("/");
+    });
+});