test(landing): guard _redirects against catalog drift (#575)

Exports a pure buildRedirects() from generate-redirects.mjs and adds a unit test asserting the committed apps/landing/public/_redirects matches it, so tool additions can't silently leave the generated redirects stale (see #573).

tests/unit/landing/redirects-drift.test.ts

@@ -0,0 +1,24 @@
+import { readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+import {
+  buildRedirects,
+  REDIRECTS_PATH,
+} from "../../../apps/landing/scripts/generate-redirects.mjs";
+
+/**
+ * Drift guard: apps/landing/public/_redirects is generated from the shared
+ * TOOLS catalog by scripts/generate-redirects.mjs (runs in the landing
+ * prebuild) but is committed, so it silently goes stale whenever a tool is
+ * added without regenerating it (that is how remove-gif-background's redirects
+ * went missing, PR #573). Adding a tool touches @snapotter/shared and runs this
+ * suite, so the drift fails here at PR time instead of shipping.
+ */
+describe("landing _redirects drift", () => {
+  it("committed public/_redirects matches the generator output", () => {
+    const committed = readFileSync(REDIRECTS_PATH, "utf8");
+    expect(
+      committed,
+      "apps/landing/public/_redirects is stale. Regenerate it with `pnpm --filter @snapotter/landing prebuild` and commit the file.",
+    ).toBe(buildRedirects());
+  });
+});