app: fixture-mode session screen and a "Run benchmark" control

BenchFixture.kt/BenchNetwork.kt fake the backend for the bench build: a
URLStreamHandlerFactory installed only under BuildConfig.FIXTURE_MODE
answers TranscriptSource/EventStream's requests from an in-memory copy of
the bundled fixture instead of opening a socket, so the fold, the paging
and uniqueItems under test are the screen's real ones rather than a
shortcut built for this. MainActivity opens straight onto that session
when FIXTURE_MODE is set, with no enrollment and no permission prompts.

BenchRun.kt drives the same scroll loop and streaming phase
transcript-bench.sh/stream-bench.sh drive over ui-trace, but in-process
(24 swipes through the real LazyListState, then 400 fixture events
appended at 20/s through the real live-fold path), and adds process CPU
time, peak RSS and battery current to the render report -- "unavailable"
rather than a fabricated number where the device can't answer.

"Run benchmark" sits beside the existing "Copy" in session settings,
found by that exact label the way every other control here is
(SessionSettingsDialog's onRunBenchmark, null on every build but bench).
debugReport gained an optional `extra` section for this; empty and
invisible on every other build's report.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

app/androidApp/src/main/kotlin/com/example/aiapp/SessionSettingsDialog.kt

@@ -66,6 +66,13 @@ fun SessionSettingsDialog(
      * measures is that screen's own state.
      */
     onCopyRenderReport: () -> Unit,
+    /**
+     * Runs P0's scripted scroll-and-stream benchmark and copies the extended report, or null on
+     * every build but `bench` -- see [BuildConfig.FIXTURE_MODE] and BenchRun.kt. Null rather than
+     * always-present-but-disabled: this has no meaning at all outside the bench build, and a
+     * control with nothing behind it on every other build is not a state worth drawing.
+     */
+    onRunBenchmark: (() -> Unit)? = null,
 ) {
     val scope = rememberCoroutineScope()
     var name by remember(sessionId) { mutableStateOf(title) }
@@ -317,6 +324,21 @@ fun SessionSettingsDialog(
                     Text("Render timings", modifier = Modifier.weight(1f))
                     TextButton(onClick = onCopyRenderReport) { Text("Copy") }
                 }
+                // Bench-build only: see [onRunBenchmark]. Named exactly "Run benchmark" because
+                // ui-trace and the emulator smoke run find it by that label, the same way every
+                // other control here is found -- see AGENTS.md's "Driving the UI".
+                onRunBenchmark?.let { run ->
+                    Spacer(Modifier.height(8.dp))
+                    Row(
+                        verticalAlignment = Alignment.CenterVertically,
+                        modifier = Modifier.fillMaxWidth(),
+                    ) {
+                        Glyph(SPEED_GLYPH, colour = MaterialTheme.colorScheme.onSurface)
+                        Spacer(Modifier.width(8.dp))
+                        Text("P0 benchmark", modifier = Modifier.weight(1f))
+                        TextButton(onClick = run) { Text("Run benchmark") }
+                    }
+                }
             }
         },
         // Disabled rather than absent while there is nothing to save: a button that comes and goes