Add NIP-19 (npub/nsec), NIP-05 verification, and arbitrary-pubkey profile fetch

- nip19.dart: bech32 encodeNpub/encodeNsec for raw keys (no TLV entities),
  verified against the canonical NIP-19 test vectors.
- nip05.dart: verifyNip05 with an injectable http.Client plus a pure
  nip05MatchesPubkey helper for the .well-known/nostr.json check.
- fetchProfileFor(fetcher, pubkey): fetches and parses any user's kind-0 into
  a typed NostrProfile (newest-wins), generalizing the own-key FetchProfile.
- Export all three from swarm.dart; unit tests for each (bech32 vectors,
  MockClient HTTP cases, fake-relay profile parsing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/nostr_profile_fetch_test.dart

@@ -0,0 +1,79 @@
+import 'dart:convert';
+
+import 'package:swarm/swarm.dart';
+import 'package:test/test.dart';
+
+import 'fake_relay_connection.dart';
+
+Map<String, dynamic> _kind0(String id, int createdAt, Map<String, dynamic> profile) => {
+      'id': id,
+      'pubkey': 'pub',
+      'created_at': createdAt,
+      'kind': 0,
+      'tags': [],
+      'content': jsonEncode(profile),
+      'sig': 'sig',
+    };
+
+void main() {
+  group('fetchProfileFor', () {
+    test('parses the kind-0 content into a NostrProfile (incl. nip05)', () async {
+      final fake = FakeRelayConnection();
+      addTearDown(fake.dispose);
+      fake.onReq = (subId) {
+        fake.push(jsonEncode([
+          'EVENT',
+          subId,
+          _kind0('e1', 100, {
+            'display_name': 'Alice',
+            'picture': 'https://cdn/x.jpg',
+            'nip05': 'alice@example.com',
+          }),
+        ]));
+        fake.push(jsonEncode(['EOSE', subId]));
+      };
+      final fetcher = NostrFetchActor(
+        relayUrls: ['ws://relay1'],
+        connectionFactory: (_) => fake,
+      );
+
+      final profile = await fetchProfileFor(fetcher, 'pub');
+
+      expect(profile, isNotNull);
+      expect(profile!.displayName, equals('Alice'));
+      expect(profile.picture, equals('https://cdn/x.jpg'));
+      expect(profile.nip05, equals('alice@example.com'));
+    });
+
+    test('picks the newest profile when several are returned', () async {
+      final fake = FakeRelayConnection();
+      addTearDown(fake.dispose);
+      fake.onReq = (subId) {
+        fake.push(jsonEncode(
+            ['EVENT', subId, _kind0('old', 100, {'display_name': 'Old'})]));
+        fake.push(jsonEncode(
+            ['EVENT', subId, _kind0('new', 200, {'display_name': 'New'})]));
+        fake.push(jsonEncode(['EOSE', subId]));
+      };
+      final fetcher = NostrFetchActor(
+        relayUrls: ['ws://relay1'],
+        connectionFactory: (_) => fake,
+      );
+
+      final profile = await fetchProfileFor(fetcher, 'pub');
+      expect(profile!.displayName, equals('New'));
+    });
+
+    test('returns null when no profile exists', () async {
+      final fake = FakeRelayConnection();
+      addTearDown(fake.dispose);
+      fake.onReq = (subId) => fake.push(jsonEncode(['EOSE', subId]));
+      final fetcher = NostrFetchActor(
+        relayUrls: ['ws://relay1'],
+        connectionFactory: (_) => fake,
+      );
+
+      expect(await fetchProfileFor(fetcher, 'pub'), isNull);
+    });
+  });
+}