feat: read subscriptions from an Atom feed when a page lists no dated links

app/test/parser_test.dart

@@ -240,4 +240,44 @@ void main() {
     expect(quoteText, hasLength(3));
     expect((quoteText[0] as MewsText).text, 'Quoted words ');
   });
+
+  group('the atom feed link', () {
+    String pageWith(String link) => '''
+<?xml version="1.0" encoding="UTF-8"?>
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>Signal Bars</title>
+    <meta name="mews-profile" content="0.1" />
+    $link
+  </head>
+  <body><h1>Signal Bars</h1></body>
+</html>
+''';
+
+    test('resolves against the page, so a relative href is usable', () {
+      final page = parseMewsPage(
+        source: pageWith(
+            '<link rel="alternate" type="application/atom+xml" href="feed.xml" />'),
+        url: 'https://example.com/posts/index.html',
+      );
+      expect(page.atomFeedHref, 'https://example.com/posts/feed.xml');
+    });
+
+    test('is refused when it points at another site', () {
+      // A client must not load another site's resources on its own
+      // (specs 7.4 and 8.1).
+      final page = parseMewsPage(
+        source: pageWith(
+            '<link rel="alternate" type="application/atom+xml" href="https://elsewhere.test/feed.xml" />'),
+        url: 'https://example.com/index.html',
+      );
+      expect(page.atomFeedHref, isNull);
+    });
+
+    test('is null when the page declares none', () {
+      final page = parseMewsPage(
+          source: pageWith(''), url: 'https://example.com/index.html');
+      expect(page.atomFeedHref, isNull);
+    });
+  });
 }