feat: add a headless entry point that runs a test with no UI

Separation that nothing exercises is separation that quietly stops being true,
so this runs a full test through Iperf3Service and Iperf3Session with
WidgetsFlutterBinding but no runApp: no widget tree, no render surface. If the
run path ever starts needing the UI, this stops working. An architecture test
additionally forbids it importing lib/ui.

It is also the accurate way to measure. A 4-stream run against a local server
reports 30.2% local CPU here, against 79.9% for a comparable run with the
transcript repainting alongside — the UI was costing roughly fifty points of
CPU on a phone already near CPU-bound at gigabit rates. Throughput figures
taken through the UI are therefore pessimistic, and this is the path to use
when the number matters.

Parameters come from --dart-define so a run needs no code edit:
  flutter run -t lib/main_headless.dart --dart-define=IPERF_SERVER=…

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/main_headless.dart

@@ -0,0 +1,105 @@
+// Headless entry point: runs one iperf3 test with no widgets.
+//
+// This exists to keep the core honest. If anything in the run path ever starts
+// depending on the UI, this stops compiling or stops working — a claim about
+// separation that is checked rather than asserted.
+//
+// It is also the low-noise way to measure. The UI competes with the test for
+// CPU (a gigabit run can put the phone near CPU-bound), so figures taken here
+// are cleaner than figures taken with a transcript repainting alongside.
+//
+//   flutter run -t lib/main_headless.dart \
+//     --dart-define=IPERF_SERVER=192.168.2.182 \
+//     --dart-define=IPERF_STREAMS=4
+//
+// Nothing below imports lib/ui, and test/architecture_test.dart enforces that.
+
+import 'package:flutter/widgets.dart' show WidgetsFlutterBinding;
+import 'package:flutter/foundation.dart' show debugPrint;
+
+import 'package:iperf3gui/data/services/iperf3_service.dart';
+import 'package:iperf3gui/domain/models/iperf3_event.dart';
+import 'package:iperf3gui/domain/models/iperf3_session.dart';
+import 'package:iperf3gui/domain/models/test_configuration.dart';
+import 'package:iperf3gui/domain/services/iperf3_runner.dart';
+
+/// Test parameters, supplied at build time with --dart-define.
+const String _server = String.fromEnvironment('IPERF_SERVER',
+    defaultValue: 'iperf.es.net');
+const int _port = int.fromEnvironment('IPERF_PORT', defaultValue: 5201);
+const int _seconds = int.fromEnvironment('IPERF_SECONDS', defaultValue: 10);
+const int _streams = int.fromEnvironment('IPERF_STREAMS', defaultValue: 1);
+
+Future<void> main() async {
+  // Required for the MethodChannel. Notably NOT runApp() — there is no widget
+  // tree, no render surface and no Flutter UI thread work in this process.
+  WidgetsFlutterBinding.ensureInitialized();
+
+  const config = TestConfiguration(
+    server: _server,
+    port: _port,
+    duration: Duration(seconds: _seconds),
+    parallelStreams: _streams,
+  );
+
+  final Iperf3Runner runner = Iperf3Service();
+  final session = Iperf3Session();
+
+  final subscription = runner.events.listen((event) {
+    session.add(event);
+    if (event is Iperf3Interval && !event.omitted) {
+      _out('interval ${event.start.toStringAsFixed(2)}-'
+          '${event.end.toStringAsFixed(2)}s  '
+          '${_mbits(event.bitsPerSecond)} Mbit/s'
+          '${event.retransmits != null ? '  retr=${event.retransmits}' : ''}');
+    } else if (event is Iperf3Error) {
+      _out('error: ${event.message}');
+    }
+  });
+
+  _out('running iperf3 -c ${config.server} -p ${config.port} '
+      '-t ${config.duration.inSeconds} -P ${config.parallelStreams}');
+
+  final result = await runner.run(config);
+  await subscription.cancel();
+  await runner.dispose();
+
+  _report(result.exitCode, session);
+}
+
+void _report(int exitCode, Iperf3Session session) {
+  _out('--- result ---');
+  _out('exit code:    $exitCode');
+  _out('samples:      ${session.sampleCount}');
+
+  final mean = session.meanBitsPerSecond;
+  final peak = session.peakBitsPerSecond;
+  if (mean != null) _out('mean:         ${_mbits(mean)} Mbit/s');
+  if (peak != null) _out('peak:         ${_mbits(peak)} Mbit/s');
+
+  final retr = session.totalRetransmits;
+  _out('retransmits:  ${retr ?? 'not measured'}');
+
+  final sent = session.summary?.sent;
+  final received = session.summary?.received;
+  if (sent != null) {
+    _out('sent:         ${_mbits(sent.bitsPerSecond)} Mbit/s '
+        'over ${sent.seconds.toStringAsFixed(2)}s');
+  }
+  if (received != null) {
+    _out('received:     ${_mbits(received.bitsPerSecond)} Mbit/s '
+        'over ${received.seconds.toStringAsFixed(2)}s');
+  }
+  final cpu = session.summary?.hostCpuPercent;
+  if (cpu != null) _out('local cpu:    ${cpu.toStringAsFixed(1)}%');
+
+  if (session.stopped) _out('ended early:  stopped');
+  for (final error in session.errors) {
+    _out('error:        $error');
+  }
+}
+
+String _mbits(double bitsPerSecond) =>
+    (bitsPerSecond / 1e6).toStringAsFixed(1);
+
+void _out(String line) => debugPrint('[iperf3-headless] $line');

test/architecture_test.dart

@@ -82,6 +82,22 @@ void main() {
       );
     });
 
+    test('the headless entry point does not depend on ui', () {
+      // main_headless.dart is the standing proof that the core runs without a
+      // widget tree; an import from lib/ui would quietly end that.
+      final offenders = _importsMatching(
+        'lib',
+        (import) => import.contains('/ui/'),
+        onlyFiles: {'lib/main_headless.dart'},
+      );
+      expect(
+        offenders,
+        isEmpty,
+        reason: 'The headless entry point must run the core with no UI.\n'
+            '${_render(offenders)}',
+      );
+    });
+
     test('every layer directory is actually covered by these rules', () {
       // Guards against a new top-level layer appearing and silently escaping
       // every rule above.
@@ -102,8 +118,9 @@ void main() {
 /// Every `(file, import)` pair under [directory] whose import satisfies [test].
 List<({String file, String import})> _importsMatching(
   String directory,
-  bool Function(String import) test,
+  bool Function(String import) test, {
-) {
+  Set<String>? onlyFiles,
+}) {
   final results = <({String file, String import})>[];
   final dir = Directory(directory);
   if (!dir.existsSync()) return results;
@@ -111,6 +128,7 @@ List<({String file, String import})> _importsMatching(
   final pattern = RegExp(r'''^\s*import\s+['"]([^'"]+)['"]''', multiLine: true);
   for (final entity in dir.listSync(recursive: true)) {
     if (entity is! File || !entity.path.endsWith('.dart')) continue;
+    if (onlyFiles != null && !onlyFiles.contains(entity.path)) continue;
     final source = entity.readAsStringSync();
     for (final match in pattern.allMatches(source)) {
       final import = match.group(1)!;