spike(block2): de-risk social layer — key derivation, OfferTransport, NIP-99 round-trip

Throwaway research spike (spike/ branch, outside pub workspace, no prod deps
touched). Validates the open Block 2 decisions before committing a funded round:

1. Deterministic one-way secp256k1 (Nostr) key derived from the Ğ1 root seed
   via HKDF — user still backs up ONE thing. Reproducible + signs (BIP340).
2. OfferTransport abstraction over Nostr NIP-99 (kind 30402); Offer stays
   inventory/location-agnostic, geohash coarsened on the wire (tested).
3. publish->discover-by-geohash proven end-to-end against an in-process
   hermetic mini-relay (~34ms round-trip).

Findings + risks + recommendation in docs/design/spike-block2-findings.md.
Block 1 suite untouched. 14 tests green, analyzer clean.

spike/block2_spike/lib/src/nostr_event.dart

@@ -0,0 +1,77 @@
+import 'dart:convert';
+
+import 'package:bip340/bip340.dart' as bip340;
+import 'package:crypto/crypto.dart';
+
+/// A minimal Nostr event (NIP-01): enough to build, id, sign and serialise
+/// NIP-99 classified listings. Throwaway — production would use a vetted client.
+class NostrEvent {
+  NostrEvent({
+    required this.pubkey,
+    required this.createdAt,
+    required this.kind,
+    required this.tags,
+    required this.content,
+    this.id = '',
+    this.sig = '',
+  });
+
+  final String pubkey; // x-only hex
+  final int createdAt; // unix seconds
+  final int kind;
+  final List<List<String>> tags;
+  final String content;
+  String id;
+  String sig;
+
+  /// NIP-01 id: sha256 of the compact JSON array
+  /// `[0, pubkey, created_at, kind, tags, content]`.
+  String computeId() {
+    final serialized = jsonEncode([0, pubkey, createdAt, kind, tags, content]);
+    final digest = sha256.convert(utf8.encode(serialized));
+    return digest.toString(); // lower-case hex
+  }
+
+  /// Fills [id] and [sig] (BIP340 Schnorr over the id) with [privateKeyHex].
+  void signWith(String privateKeyHex, {String? auxRandHex}) {
+    id = computeId();
+    // Deterministic aux in the spike (derived from id) so round-trip tests are
+    // reproducible; production uses fresh randomness per BIP340.
+    final aux = auxRandHex ?? id;
+    sig = bip340.sign(privateKeyHex, id, aux);
+  }
+
+  /// Verifies the signature against [pubkey]. Cross-checks that a real relay /
+  /// peer would accept what we produced.
+  bool verify() => id == computeId() && bip340.verify(pubkey, id, sig);
+
+  Map<String, dynamic> toJson() => {
+    'id': id,
+    'pubkey': pubkey,
+    'created_at': createdAt,
+    'kind': kind,
+    'tags': tags,
+    'content': content,
+    'sig': sig,
+  };
+
+  static NostrEvent fromJson(Map<String, dynamic> j) => NostrEvent(
+    id: j['id'] as String,
+    pubkey: j['pubkey'] as String,
+    createdAt: j['created_at'] as int,
+    kind: j['kind'] as int,
+    tags: (j['tags'] as List)
+        .map((t) => (t as List).map((e) => e as String).toList())
+        .toList(),
+    content: j['content'] as String,
+    sig: j['sig'] as String,
+  );
+
+  /// First value of the first tag named [name], or null.
+  String? tag(String name) {
+    for (final t in tags) {
+      if (t.isNotEmpty && t[0] == name && t.length > 1) return t[1];
+    }
+    return null;
+  }
+}