spike(block2): de-risk web of trust — the last big unknown

Adds TrustTransport as the third interface on the shared NostrConnection,
completing the social-layer happy path in the spike:

- WebOfTrust: pure, Flutter-free Duniter membership rule (N certs from members +
  within distance D of bootstrap referents), iterated to a fixpoint. The
  commons_core-worthy piece; TDD'd (threshold, distance cutoff, transitive
  growth, expired/revoked/self exclusion).
- NostrTrustTransport: certifications as a custom addressable kind (30777,
  keyed by issuer+subject) — certify renews, revoke replaces, certs expire.
- Trust filters spam (network-trust §2): seeds certify Alice; Eve stays
  uncertified; Bob discovers both offers and annotates authors — Alice known,
  Eve unknown — all over ONE shared connection.

Findings updated: WoT risk High -> Medium (policy/bootstrap, not feasibility);
overall scope Medium-High -> Medium — all three transport contracts now proven.
30 tests green, analyzer clean, Block 1 untouched.

spike/block2_spike/lib/src/nostr_trust_transport.dart

@@ -0,0 +1,97 @@
+import 'nostr_connection.dart';
+import 'nostr_event.dart';
+import 'trust_transport.dart';
+import 'web_of_trust.dart';
+
+/// Duniter-style certifications carried as Nostr events, on the shared
+/// [NostrConnection]. There is no settled NIP for a web of trust (NIP-85 is
+/// unsettled), so the spike uses a **custom addressable kind** mapped to the
+/// Duniter model — exactly the "own WoT, Duniter-compatible" path
+/// g1-integration.md §"WoT propia pero compatible" anticipates.
+///
+/// Addressable by (kind, issuer, `d`=subject): one live certification per
+/// (certifier, subject), so re-certifying renews and revoking replaces — the
+/// relay keeps only the latest, like Ğ1 keeps one active cert per pair.
+class NostrTrustTransport implements TrustTransport {
+  NostrTrustTransport(this._conn);
+
+  final NostrConnection _conn;
+
+  /// Custom Tanemaki certification kind (addressable range).
+  static const kindCertification = 30777;
+
+  int get _now => DateTime.now().millisecondsSinceEpoch ~/ 1000;
+
+  @override
+  Future<void> certify({
+    required String subjectPubkey,
+    Duration validity = const Duration(days: 365),
+    String note = '',
+  }) =>
+      _publishCert(subjectPubkey, note: note, validity: validity, revoked: false);
+
+  @override
+  Future<void> revoke({required String subjectPubkey}) =>
+      _publishCert(subjectPubkey, note: '', validity: Duration.zero, revoked: true);
+
+  Future<void> _publishCert(
+    String subject, {
+    required String note,
+    required Duration validity,
+    required bool revoked,
+  }) async {
+    final event = NostrEvent(
+      pubkey: _conn.publicKeyHex,
+      createdAt: _now,
+      kind: kindCertification,
+      tags: [
+        ['p', subject],
+        ['d', subject], // one addressable cert per (issuer, subject)
+        ['status', revoked ? 'revoked' : 'valid'],
+        if (!revoked) ['expiration', '${_now + validity.inSeconds}'],
+      ],
+      content: note,
+    )..signWith(_conn.privateKeyHex);
+    final r = await _conn.publish(event);
+    if (!r.accepted) throw StateError('relay rejected cert: ${r.message}');
+  }
+
+  @override
+  Future<List<Certification>> allCertifications() async {
+    final events = await _conn.reqOnce({
+      'kinds': [kindCertification],
+    });
+    return events.map(_toCertification).toList();
+  }
+
+  @override
+  Future<Set<String>> certifiersOf(String subjectPubkey) async {
+    final events = await _conn.reqOnce({
+      'kinds': [kindCertification],
+      '#p': [subjectPubkey],
+    });
+    final now = DateTime.now();
+    return events
+        .map(_toCertification)
+        .where((c) => c.subject == subjectPubkey && c.isValidAt(now))
+        .map((c) => c.issuer)
+        .toSet();
+  }
+
+  Certification _toCertification(NostrEvent e) {
+    final exp = e.tag('expiration');
+    return Certification(
+      issuer: e.pubkey,
+      subject: e.tag('p') ?? e.tag('d') ?? '',
+      issuedAt: DateTime.fromMillisecondsSinceEpoch(e.createdAt * 1000),
+      expiresAt: exp == null
+          ? null
+          : DateTime.fromMillisecondsSinceEpoch(int.parse(exp) * 1000),
+      revoked: e.tag('status') == 'revoked',
+      note: e.content,
+    );
+  }
+
+  @override
+  Future<void> close() => _conn.close();
+}