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/error.test.tsx

@@ -0,0 +1,13 @@
+import { describe, expect, it, vi } from "vitest";
+import { fireEvent, render, screen } from "@testing-library/react";
+import ErrorPage from "@/app/error";
+
+describe("Error page", () => {
+    it("renders and retries on click", () => {
+        const reset = vi.fn();
+        render(<ErrorPage error={new Error("boom")} reset={reset} />);
+        expect(screen.getByText("// 500")).toBeDefined();
+        fireEvent.click(screen.getByRole("button"));
+        expect(reset).toHaveBeenCalledOnce();
+    });
+});