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/test/roundtrip_test.dart

@@ -0,0 +1,132 @@
+import 'dart:typed_data';
+
+import 'package:block2_spike/block2_spike.dart';
+import 'package:test/test.dart';
+
+/// Q3 — publish → discover a real offer by geohash against a running relay
+/// (in-process, hermetic). Proves the flow works end to end and measures it.
+void main() {
+  late MiniRelay relay;
+
+  setUp(() async => relay = await MiniRelay.start());
+  tearDown(() async => relay.stop());
+
+  NostrKey keyFor(int fill) =>
+      NostrKey.deriveFromSeed(Uint8List(32)..fillRange(0, 32, fill));
+
+  test('offer published by one identity is discovered by another via geohash',
+      () async {
+    final alice = keyFor(1);
+    final bob = keyFor(2);
+
+    final aliceTransport = await NostrOfferTransport.connect(
+      relay.url,
+      privateKeyHex: alice.privateKeyHex,
+      pubkeyHex: alice.publicKeyHex,
+    );
+    final bobTransport = await NostrOfferTransport.connect(
+      relay.url,
+      privateKeyHex: bob.privateKeyHex,
+      pubkeyHex: bob.publicKeyHex,
+    );
+
+    final offer = Offer(
+      id: 'tomate-1',
+      authorPubkeyHex: alice.publicKeyHex,
+      summary: 'Tomate rosa de Barbastro',
+      type: OfferType.gift,
+      category: 'Solanaceae',
+      approxGeohash: 'sp3e9xyz', // will be coarsened to sp3e9
+    );
+
+    final published = await aliceTransport.publish(offer);
+    expect(published.accepted, isTrue,
+        reason: 'relay OK: ${published.message}');
+
+    // Bob queries a coarser area and still finds it (ladder match).
+    final found = await bobTransport.discoverUntilEose(
+      const DiscoveryQuery(geohashPrefix: 'sp3'),
+    );
+    expect(found, hasLength(1));
+    expect(found.single.summary, 'Tomate rosa de Barbastro');
+    expect(found.single.authorPubkeyHex, alice.publicKeyHex);
+
+    await aliceTransport.close();
+    await bobTransport.close();
+  });
+
+  test('a query for a different area returns nothing (geohash scoping works)',
+      () async {
+    final alice = keyFor(1);
+    final t = await NostrOfferTransport.connect(
+      relay.url,
+      privateKeyHex: alice.privateKeyHex,
+      pubkeyHex: alice.publicKeyHex,
+    );
+    await t.publish(Offer(
+      id: 'x',
+      authorPubkeyHex: alice.publicKeyHex,
+      summary: 'Pimiento',
+      type: OfferType.gift,
+      approxGeohash: 'sp3e9',
+    ));
+    final elsewhere = await t.discoverUntilEose(
+      const DiscoveryQuery(geohashPrefix: 'u09'),
+    );
+    expect(elsewhere, isEmpty);
+    await t.close();
+  });
+
+  test('re-publishing the same offer id replaces (addressable), not duplicates',
+      () async {
+    final alice = keyFor(3);
+    final t = await NostrOfferTransport.connect(
+      relay.url,
+      privateKeyHex: alice.privateKeyHex,
+      pubkeyHex: alice.publicKeyHex,
+    );
+    Offer o(String summary) => Offer(
+          id: 'lot-42',
+          authorPubkeyHex: alice.publicKeyHex,
+          summary: summary,
+          type: OfferType.exchange,
+          exchangeTerms: 'a cambio de legumbre',
+          approxGeohash: 'sp3e9',
+        );
+    await t.publish(o('v1'));
+    await t.publish(o('v2'));
+    final found = await t.discoverUntilEose(
+      const DiscoveryQuery(geohashPrefix: 'sp3e9'),
+    );
+    expect(found, hasLength(1), reason: 'addressable replace by (kind,pubkey,d)');
+    expect(found.single.summary, 'v2');
+    await t.close();
+  });
+
+  test('measures publish→discover latency (informational)', () async {
+    final alice = keyFor(4);
+    final t = await NostrOfferTransport.connect(
+      relay.url,
+      privateKeyHex: alice.privateKeyHex,
+      pubkeyHex: alice.publicKeyHex,
+    );
+    final sw = Stopwatch()..start();
+    await t.publish(Offer(
+      id: 'latency',
+      authorPubkeyHex: alice.publicKeyHex,
+      summary: 'Calabaza',
+      type: OfferType.gift,
+      approxGeohash: 'sp3e9',
+    ));
+    final found = await t.discoverUntilEose(
+      const DiscoveryQuery(geohashPrefix: 'sp3e9'),
+    );
+    sw.stop();
+    expect(found, hasLength(1));
+    // Not an assertion on wall-time (CI is noisy); just surface the number.
+    printOnFailure('publish+discover round-trip: ${sw.elapsedMilliseconds} ms');
+    // ignore: avoid_print
+    print('[metric] publish+discover round-trip: ${sw.elapsedMilliseconds} ms');
+    await t.close();
+  });
+}