Use the glossary's words in test names and comments (review item 2)

"Feed" no longer stands for a Source and "newsletter" no longer for a
Learning source; paid_newsletter_posts_test.dart becomes
paid_substack_posts_test.dart.

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

test/support/fake_app.dart

@@ -14,7 +14,7 @@ class FakeFeeds extends FeedService {
   /// Feed URL -> (title, age) of each article in the feed.
   final Map<String, List<(String, Duration)>> byFeedUrl;
 
-  /// How often the app refreshed its feeds.
+  /// How often the app refreshed all Sources.
   int refreshes = 0;
 
   @override