Add NIP-17 gift-wrapped direct messages

Add outbound NIP-17 private messaging: build the kind-14 rumor -> kind-13
seal -> kind-1059 gift wrap chain on the existing NIP-44 primitives, wrapped
in NostrDmActor (WrapDirectMessage/GiftWrapped). Publishing is left to
NostrPublishActor.

Extract nostrEventId() from signNostrEvent so the unsigned rumor can compute
its id without a signature. Export the new actor and nip17 helpers. Add a
round-trip test (wrap -> unwrap) and a negative decryption test.

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

test/nostr_dm_actor_test.dart

@@ -0,0 +1,71 @@
+import 'dart:convert';
+
+import 'package:bip340/bip340.dart' as bip340;
+import 'package:swarm/swarm.dart';
+import 'package:test/test.dart';
+
+const _senderPriv =
+    '0000000000000000000000000000000000000000000000000000000000000001';
+const _recipientPriv =
+    '0000000000000000000000000000000000000000000000000000000000000002';
+
+void main() {
+  final senderPub = bip340.getPublicKey(_senderPriv);
+  final recipientPub = bip340.getPublicKey(_recipientPriv);
+
+  test('WrapDirectMessage produces a kind-1059 gift wrap', () async {
+    final actor = NostrDmActor(_senderPriv);
+    final result = await actor.handle(
+      WrapDirectMessage(recipientPubKey: recipientPub, content: 'hi'),
+    );
+    expect(result, isA<GiftWrapped>());
+    final event =
+        jsonDecode((result as GiftWrapped).event) as Map<String, dynamic>;
+    expect(event['kind'], kGiftWrapKind);
+    // The wrap is signed by an ephemeral key, never the sender.
+    expect(event['pubkey'], isNot(equals(senderPub)));
+    final pTags = (event['tags'] as List)
+        .cast<List>()
+        .where((t) => t.isNotEmpty && t[0] == 'p');
+    expect(pTags.single[1], equals(recipientPub));
+  });
+
+  test('recipient unwraps the gift back to the original rumor', () async {
+    final actor = NostrDmActor(_senderPriv);
+    final result = await actor.handle(
+      WrapDirectMessage(
+        recipientPubKey: recipientPub,
+        content: 'check out this image',
+        rumorTags: const [
+          ['imeta', 'url https://cdn.example/x.png', 'm image/png'],
+        ],
+      ),
+    ) as GiftWrapped;
+
+    final rumor = unwrapGift(result.event, _recipientPriv);
+    expect(rumor['content'], equals('check out this image'));
+    expect(rumor['kind'], equals(kDirectMessageKind));
+    expect(rumor['pubkey'], equals(senderPub));
+
+    final tags = (rumor['tags'] as List).cast<List>();
+    expect(
+      tags.any((t) => t.isNotEmpty && t[0] == 'p' && t[1] == recipientPub),
+      isTrue,
+    );
+    expect(
+      tags.any((t) => t.isNotEmpty && t[0] == 'imeta'),
+      isTrue,
+    );
+  });
+
+  test('a different key cannot unwrap the gift', () async {
+    final actor = NostrDmActor(_senderPriv);
+    final result = await actor.handle(
+      WrapDirectMessage(recipientPubKey: recipientPub, content: 'secret'),
+    ) as GiftWrapped;
+
+    const otherPriv =
+        '0000000000000000000000000000000000000000000000000000000000000003';
+    expect(() => unwrapGift(result.event, otherPriv), throwsA(anything));
+  });
+}