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/project.json

@@ -39,6 +39,16 @@
                 "command": "pnpm exec playwright test --config=playwright.performance.config.ts src/m3u-import.performance.ts"
             }
         },
+        "benchmark-xtream": {
+            "dependsOn": ["electron-backend:build-performance"],
+            "executor": "nx:run-commands",
+            "cache": false,
+            "parallelism": false,
+            "options": {
+                "cwd": "apps/electron-backend-e2e",
+                "command": "pnpm exec playwright test --config=playwright.xtream-performance.config.ts src/xtream.performance.ts"
+            }
+        },
         "packaged-frame-copy-smoke": {
             "dependsOn": ["test-packaged-frame-copy-fixtures"],
             "executor": "nx:run-commands",