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/nip05_test.dart

@@ -0,0 +1,89 @@
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+import 'package:swarm/swarm.dart';
+import 'package:test/test.dart';
+
+const _pubkey =
+    '7e7e9c42a91bfef19fa929e5fda1b72e0ebc1a4c1141673e2794234d86addf4e';
+
+void main() {
+  group('nip05MatchesPubkey (pure)', () {
+    test('true when names maps the name to the pubkey', () {
+      final body = jsonEncode({
+        'names': {'alice': _pubkey},
+      });
+      expect(nip05MatchesPubkey(body, 'alice', _pubkey), isTrue);
+    });
+
+    test('case-insensitive on the pubkey', () {
+      final body = jsonEncode({
+        'names': {'alice': _pubkey.toUpperCase()},
+      });
+      expect(nip05MatchesPubkey(body, 'alice', _pubkey), isTrue);
+    });
+
+    test('false on name mismatch, missing name, or junk', () {
+      final body = jsonEncode({
+        'names': {'alice': _pubkey},
+      });
+      expect(nip05MatchesPubkey(body, 'bob', _pubkey), isFalse);
+      expect(
+        nip05MatchesPubkey(jsonEncode({'names': {}}), 'alice', _pubkey),
+        isFalse,
+      );
+      expect(nip05MatchesPubkey('not json', 'alice', _pubkey), isFalse);
+    });
+  });
+
+  group('verifyNip05 (mocked HTTP)', () {
+    test('verifies a well-formed match and queries the right URL', () async {
+      late Uri requested;
+      final client = MockClient((req) async {
+        requested = req.url;
+        return http.Response(
+          jsonEncode({
+            'names': {'alice': _pubkey},
+          }),
+          200,
+        );
+      });
+
+      final ok = await verifyNip05('alice@example.com', _pubkey,
+          client: client);
+      expect(ok, isTrue);
+      expect(requested.host, equals('example.com'));
+      expect(requested.path, equals('/.well-known/nostr.json'));
+      expect(requested.queryParameters['name'], equals('alice'));
+    });
+
+    test('false on name mismatch', () async {
+      final client = MockClient((req) async => http.Response(
+            jsonEncode({
+              'names': {'alice': 'deadbeef'},
+            }),
+            200,
+          ));
+      expect(
+        await verifyNip05('alice@example.com', _pubkey, client: client),
+        isFalse,
+      );
+    });
+
+    test('false on non-200', () async {
+      final client = MockClient((req) async => http.Response('', 404));
+      expect(
+        await verifyNip05('alice@example.com', _pubkey, client: client),
+        isFalse,
+      );
+    });
+
+    test('false on malformed identifier', () async {
+      final client = MockClient((req) async => http.Response('', 200));
+      expect(await verifyNip05('no-at-sign', _pubkey, client: client), isFalse);
+      expect(await verifyNip05('@example.com', _pubkey, client: client),
+          isFalse);
+    });
+  });
+}