test(performance): add end-to-end Xtream benchmark harness (#1300)

* docs(performance): plan Xtream benchmark

* feat(xtream-mock-server): add deterministic 100k fixture

* style(xtream-mock-server): apply repository formatting

* fix(xtream-mock-server): harden performance fixture data

* feat(xtream-mock-server): add performance control plane

* docs(performance): correct Xtream capture plan

* fix(xtream-mock-server): harden performance controls

* fix(xtream-mock-server): harden control lifecycle

* feat(performance): add Xtream preload markers

* feat(performance): trace Xtream main phases

* feat(performance): mark Xtream store publications

* feat(performance): trace Xtream database phases

* feat(performance): trace Xtream delete cancellation

* feat(performance): capture Xtream phase attribution

* feat(performance): mark Sources Xtream refresh

* test(performance): define Xtream benchmark evidence contracts

* test(performance): add Xtream benchmark runner

* test(performance): surface failure evidence writes

* test(performance): align database read clock

* test(performance): preserve capture failure contracts

apps/electron-backend-e2e/src/performance/xtream-benchmark-runtime.ts

@@ -0,0 +1,52 @@
+import type { Page } from '@playwright/test';
+
+import {
+    XTREAM_SCENARIO_ID,
+    type XtreamScenarioId,
+} from './xtream-benchmark-contract';
+
+export function xtreamScenarioRequiresSeed(
+    scenarioId: XtreamScenarioId
+): boolean {
+    return (
+        scenarioId === XTREAM_SCENARIO_ID.REFRESH_LARGE ||
+        scenarioId === XTREAM_SCENARIO_ID.DELETE_LARGE ||
+        scenarioId === XTREAM_SCENARIO_ID.BACKGROUND_UI
+    );
+}
+
+export function deriveXtreamMeasuredPlaylistId(
+    requests: readonly { readonly playlistId: string | null }[]
+): string {
+    const identities = new Set(
+        requests
+            .map(({ playlistId }) => playlistId)
+            .filter(
+                (playlistId): playlistId is string =>
+                    typeof playlistId === 'string' && playlistId.length > 0
+            )
+    );
+    const playlistId = identities.values().next().value as string | undefined;
+    if (identities.size !== 1 || !playlistId) {
+        throw new Error('xtream-measured-playlist-identity-invalid');
+    }
+    return playlistId;
+}
+
+export async function disableGenericElectronFixtureProbes(
+    page: Page
+): Promise<void> {
+    await page.evaluate(() => {
+        window.__portalDebugUnsubscribe?.();
+        window.__portalDebugUnsubscribe = undefined;
+        window.__portalDebugEvents = [];
+        window.__dbOperationUnsubscribe?.();
+        window.__dbOperationUnsubscribe = undefined;
+        window.__dbOperationEvents = [];
+        if (window.__rendererFrameRequestId) {
+            window.cancelAnimationFrame(window.__rendererFrameRequestId);
+        }
+        window.__rendererFrameRequestId = undefined;
+        window.__rendererFrameCount = 0;
+    });
+}