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-memory-validity.ts

@@ -0,0 +1,13 @@
+export function assertXtreamWorkerHeapWithinMainRss(
+    worker: unknown,
+    main: unknown
+): void {
+    const workerMetrics = worker as Record<string, unknown>;
+    const mainMetrics = main as Record<string, unknown>;
+    if (
+        (workerMetrics['peakHeapUsedBytes'] as number) >
+        (mainMetrics['peakRssBytes'] as number)
+    ) {
+        throw new Error('database-worker heap exceeds Electron main RSS');
+    }
+}