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>

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)!;