test(player): pin the native-video default and the opt-out that must survive it

The default has moved four times, so the risk is not which way it points but
that a flip silently overrides people who chose. The previous reader was
getItem(KEY) === "true", which conflates "never chose" with "chose off" — under
it, flipping the default re-enables the native path for everyone who had
deliberately turned it off. The three cases are pinned separately so that
conflation cannot come back.

src/lib/stores/nativeVideo.default.test.ts

@@ -0,0 +1,75 @@
+import { describe, it, expect, beforeEach, beforeAll, afterAll, vi } from "vitest";
+import { get } from "svelte/store";
+
+/**
+ * The stored value of the native-video preference, and what it means.
+ *
+ * The default has moved four times (see the history on `load()` in
+ * nativeVideo.ts), so the risk here is not "which way is it pointing" — it is
+ * that a flip silently overrides people who chose. The old reader was
+ * `getItem(KEY) === "true"`, which conflates "never chose" with "chose off";
+ * flipping the default under that reader re-enables the native path for
+ * everyone who deliberately turned it off. So the three cases are pinned
+ * separately rather than through the default alone.
+ *
+ * TRACES: UR-003, UR-004 | DR-188
+ */
+
+const STORAGE_KEY = "jellytau-experimental-native-video";
+
+// jsdom here doesn't expose localStorage; stand in a minimal implementation,
+// matching the viewMode/searchGroupOrder store tests.
+const backing = new Map<string, string>();
+const localStorageShim = {
+  getItem: (key: string) => backing.get(key) ?? null,
+  setItem: (key: string, value: string) => void backing.set(key, value),
+  removeItem: (key: string) => void backing.delete(key),
+  clear: () => backing.clear(),
+};
+
+beforeAll(() => {
+  vi.stubGlobal("localStorage", localStorageShim);
+});
+
+afterAll(() => {
+  vi.unstubAllGlobals();
+});
+
+async function freshStore() {
+  // The default is read at module init, so each case needs a fresh module.
+  vi.resetModules();
+  return await import("./nativeVideo");
+}
+
+describe("experimentalNativeVideo default", () => {
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  it("defaults to ON when the user has never chosen", async () => {
+    const { experimentalNativeVideo } = await freshStore();
+    expect(get(experimentalNativeVideo)).toBe(true);
+  });
+
+  it("stays OFF for someone who deliberately turned it off", async () => {
+    // The regression the null check exists for: an explicit opt-out must
+    // survive the default flip, not be re-enabled by it.
+    localStorage.setItem(STORAGE_KEY, "false");
+    const { experimentalNativeVideo } = await freshStore();
+    expect(get(experimentalNativeVideo)).toBe(false);
+  });
+
+  it("stays ON for someone who deliberately turned it on", async () => {
+    localStorage.setItem(STORAGE_KEY, "true");
+    const { experimentalNativeVideo } = await freshStore();
+    expect(get(experimentalNativeVideo)).toBe(true);
+  });
+
+  it("persists an explicit choice in both directions", async () => {
+    const { experimentalNativeVideo } = await freshStore();
+    experimentalNativeVideo.set(false);
+    expect(localStorage.getItem(STORAGE_KEY)).toBe("false");
+    experimentalNativeVideo.set(true);
+    expect(localStorage.getItem(STORAGE_KEY)).toBe("true");
+  });
+});