Keep per-kind facts on SourceKind and the Briefing's input in one place (review items 4, 5, 6, 7)

- SourceKind carries whether it feeds the Briefing, its fixed time window
  (Learning, Fun) and whether reading is picture-led (Fun); windowOf and
  the reader read from it.
- AppState.briefingArticles is the one place that says only News sources
  feed the Briefing; generateBriefing and the Briefing tab use it.
- articlesOf, hiddenCountOf and briefingArticles share one filter.
- FeedService.fetchAll takes windowOf: like AppState.windowOf.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/source_kinds_test.dart

@@ -1,10 +1,13 @@
 import 'dart:convert';
 
 import 'package:daily_tech_news/data/default_sources.dart';
+import 'package:daily_tech_news/models/app_settings.dart';
 import 'package:daily_tech_news/models/article.dart';
 import 'package:daily_tech_news/models/briefing.dart';
 import 'package:daily_tech_news/models/source.dart';
 import 'package:daily_tech_news/services/summarizer.dart';
+import 'package:daily_tech_news/ui/briefing_screen.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
@@ -62,6 +65,37 @@ void main() {
     expect(_titles(summarizer.briefingInput!), ['News from this morning']);
   });
 
+  test('the Briefing reads the shown News articles, not Learning, Fun or hidden ones', () async {
+    final xkcd = testSource('xkcd', SourceKind.fun);
+    final state = await launchApp([verge, bytebytego, xkcd], feeds: {
+      verge.feedUrl: [('News from this morning', const Duration(hours: 2)),
+        ('Sponsored: Buy our phone', const Duration(hours: 3))],
+      bytebytego.feedUrl: [('Explainer from today', const Duration(hours: 1))],
+      xkcd.feedUrl: [('Comic from today', const Duration(hours: 1))],
+    });
+
+    expect(_titles(state.briefingArticles), ['News from this morning']);
+  });
+
+  testWidgets('the Briefing tab counts only the articles the Briefing is made from',
+      (tester) async {
+    final state = (await tester.runAsync(() async {
+      final state = await launchApp([verge, bytebytego], feeds: {
+        verge.feedUrl: [('News from this morning', const Duration(hours: 2))],
+        bytebytego.feedUrl: [('Explainer from today', const Duration(hours: 1)),
+          ('Explainer from yesterday', const Duration(days: 1))],
+      });
+      await state.updateSettings(
+          state.settings.copyWith(aiProvider: AiProvider.claude, claudeApiKey: 'key'));
+      return state;
+    }))!;
+
+    await tester.pumpWidget(
+        MaterialApp(home: BriefingScreen(state: state, openSettings: () {})));
+
+    expect(find.text('Generate from 1 articles'), findsOneWidget);
+  });
+
   test('News follows the window setting', () async {
     SharedPreferences.setMockInitialValues({'max_age_hours': 48});
     final state = await launchApp([verge], feeds: {

test/source_selection_test.dart

@@ -10,7 +10,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 class _NoFeeds extends FeedService {
   @override
   Future<FeedResult> fetchAll(List<Source> sources,
-          {required Duration Function(Source source) maxAgeOf, String corsProxy = ''}) async =>
+          {required Duration Function(Source source) windowOf, String corsProxy = ''}) async =>
       const FeedResult([], {});
 }
 

test/support/fake_app.dart

@@ -19,9 +19,9 @@ class FakeFeeds extends FeedService {
 
   @override
   Future<FeedResult> fetchAll(List<Source> sources,
-      {required Duration Function(Source source) maxAgeOf, String corsProxy = ''}) {
+      {required Duration Function(Source source) windowOf, String corsProxy = ''}) {
     refreshes++;
-    return super.fetchAll(sources, maxAgeOf: maxAgeOf, corsProxy: corsProxy);
+    return super.fetchAll(sources, windowOf: windowOf, corsProxy: corsProxy);
   }
 
   @override