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/privacy_test.dart

@@ -0,0 +1,80 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:block2_spike/block2_spike.dart';
+import 'package:test/test.dart';
+
+/// Q2 — the `OfferTransport` seam must not leak inventory or an exact location.
+/// The offer carries only a chosen summary; the wire event is checked byte-wise.
+void main() {
+  final key = NostrKey.deriveFromSeed(
+    Uint8List.fromList(List.generate(32, (i) => i)),
+  );
+  final codec = Nip99Codec();
+
+  // A realistic private-ish context: the caller ACCIDENTALLY passes a very
+  // precise geohash and there is secret inventory the offer must never expose.
+  const secretInventoryMarker = 'SECRET-LOT-12-KG-IN-SHED';
+  const preciseGeohash = 'u09tunqmabc'; // ~sub-metre precision
+
+  Offer sampleOffer() => Offer(
+    id: 'offer-1',
+    authorPubkeyHex: key.publicKeyHex,
+    summary: 'Tomate rosa de Barbastro',
+    type: OfferType.gift,
+    category: 'Solanaceae',
+    approxGeohash: preciseGeohash,
+  );
+
+  test('exact location is coarsened on the wire (never full precision)', () {
+    final event = codec.encode(sampleOffer(), createdAt: 1000);
+    final gTags = event.tags.where((t) => t[0] == 'g').map((t) => t[1]);
+    // No geohash tag exceeds the coarse cap...
+    expect(gTags.every((g) => g.length <= Nip99Codec.maxGeohashChars), isTrue);
+    // ...and the precise fix never appears anywhere in the serialised event.
+    final wire = jsonEncode(event.toJson());
+    expect(wire, isNot(contains(preciseGeohash)));
+    expect(wire, contains('u09t')); // coarse prefix is present
+  });
+
+  test('no inventory / address field can ride along (seam has none)', () {
+    final event = codec.encode(sampleOffer(), createdAt: 1000);
+    final wire = jsonEncode(event.toJson());
+    expect(wire, isNot(contains(secretInventoryMarker)));
+    // The event has no "location", "address" or "geo-exact" tag by construction.
+    final tagNames = event.tags.map((t) => t[0]).toSet();
+    expect(tagNames, isNot(contains('location')));
+    expect(tagNames, isNot(contains('address')));
+  });
+
+  test('geohash ladder lets an area query match by exact tag', () {
+    final event = codec.encode(sampleOffer(), createdAt: 1000);
+    final gTags = event.tags.where((t) => t[0] == 'g').map((t) => t[1]).toList();
+    expect(gTags, containsAll(['u', 'u0', 'u09', 'u09t']));
+  });
+
+  test('round-trips Offer ↔ NIP-99 event, only chosen fields survive', () {
+    final original = Offer(
+      id: 'offer-2',
+      authorPubkeyHex: key.publicKeyHex,
+      summary: 'Judía del ganxet',
+      type: OfferType.sale,
+      priceAmount: 3,
+      priceCurrency: 'G1',
+      category: 'Fabaceae',
+      approxGeohash: 'sp3e9',
+    );
+    final event = codec.encode(original, createdAt: 1000)
+      ..signWith(key.privateKeyHex);
+    expect(event.verify(), isTrue);
+
+    final decoded = codec.decode(event);
+    expect(decoded.id, original.id);
+    expect(decoded.summary, original.summary);
+    expect(decoded.type, OfferType.sale);
+    expect(decoded.priceAmount, 3);
+    expect(decoded.priceCurrency, 'G1');
+    expect(decoded.category, 'Fabaceae');
+    expect(decoded.approxGeohash, 'sp3e9');
+  });
+}