fix(app): a one-word address is refused at once; this machine's is reached over HTTP

An address of one word (`todo`) was read as https://todo/ and looked for,
the person waiting up to 15 s for no instance to answer. A host is now a
domain of two labels or more, an IP address or localhost; anything else is
"This isn't an address." as it is typed. localhost and loopback addresses
typed without a scheme are reached over HTTP, as a server running on the
same machine has no certificate; HTTPS where typed (the developer's
review, 9 Nov 2026).

Whats-New: An address of a single word is now refused at once, rather than looked for.
Whats-New-De: Eine Adresse aus nur einem Wort wird jetzt sofort abgelehnt, statt danach zu suchen.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/instance/instance_test.dart

@@ -24,6 +24,31 @@ void main() {
     );
   });
 
+  test('reaches this machine over HTTP where no scheme is typed, as a '
+      "server running on it has no certificate (the developer's review, "
+      '9 Nov 2026)', () {
+    for (final (address, url) in [
+      ('localhost:8080', 'http://localhost:8080/'),
+      ('LOCALHOST', 'http://localhost/'),
+      ('127.0.0.1:8080', 'http://127.0.0.1:8080/'),
+      ('[::1]:8080', 'http://[::1]:8080/'),
+      ('https://localhost:8443', 'https://localhost:8443/'),
+    ]) {
+      expect(Instance.parse(address)?.url, Uri.parse(url), reason: address);
+    }
+  });
+
+  test('takes an IP address as a host', () {
+    expect(
+      Instance.parse('192.168.1.10:8080')?.url,
+      Uri.parse('https://192.168.1.10:8080/'),
+    );
+    expect(
+      Instance.parse('http://[2001:db8::1]')?.url,
+      Uri.parse('http://[2001:db8::1]/'),
+    );
+  });
+
   test('refuses what is no address', () {
     for (final address in [
       '',
@@ -32,6 +57,13 @@ void main() {
       'ftp://example.org',
       'https://',
       'https://someone@example.org',
+      // A host of one word that isn't this machine, which no instance on
+      // the internet has: refused as typed rather than looked for
+      // (the developer's review, 9 Nov 2026).
+      'todo',
+      'https://learn',
+      'learn.',
+      '.org',
     ]) {
       expect(Instance.parse(address), isNull, reason: address);
     }