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/derivation_test.dart

@@ -0,0 +1,76 @@
+import 'dart:math';
+import 'dart:typed_data';
+
+import 'package:bip340/bip340.dart' as bip340;
+import 'package:block2_spike/block2_spike.dart';
+import 'package:commons_core/commons_core.dart';
+import 'package:test/test.dart';
+
+/// Q1 — Identity derivation: a deterministic, one-way secp256k1 (Nostr) key from
+/// the Duniter/Ğ1-style root seed, so the user still backs up ONE thing.
+void main() {
+  group('NostrKey.deriveFromSeed', () {
+    // A fixed seed so expectations are reproducible across runs/machines.
+    final seed = Uint8List.fromList(List.generate(32, (i) => i));
+
+    test('is reproducible: same seed → identical key', () {
+      final a = NostrKey.deriveFromSeed(seed);
+      final b = NostrKey.deriveFromSeed(seed);
+      expect(a.privateKeyHex, b.privateKeyHex);
+      expect(a.publicKeyHex, b.publicKeyHex);
+      expect(a.npub, b.npub);
+    });
+
+    test('produces a well-formed x-only key and npub/nsec', () {
+      final key = NostrKey.deriveFromSeed(seed);
+      expect(key.privateKeyHex, matches(RegExp(r'^[0-9a-f]{64}$')));
+      expect(key.publicKeyHex, matches(RegExp(r'^[0-9a-f]{64}$')));
+      // Cross-check: bip340 derives the same pubkey from our private hex.
+      expect(bip340.getPublicKey(key.privateKeyHex), key.publicKeyHex);
+      expect(key.npub, startsWith('npub1'));
+      expect(key.nsec, startsWith('nsec1'));
+    });
+
+    test('the derived key actually signs & verifies (BIP340)', () {
+      final key = NostrKey.deriveFromSeed(seed);
+      final message = 'a' * 64; // 32-byte hex message
+      final sig = bip340.sign(key.privateKeyHex, message, 'b' * 64);
+      expect(bip340.verify(key.publicKeyHex, message, sig), isTrue);
+    });
+
+    test('different seeds → different keys (avalanche on a 1-bit flip)', () {
+      final flipped = Uint8List.fromList(seed)..[0] ^= 0x01;
+      final a = NostrKey.deriveFromSeed(seed);
+      final b = NostrKey.deriveFromSeed(flipped);
+      expect(a.privateKeyHex, isNot(b.privateKeyHex));
+      expect(a.publicKeyHex, isNot(b.publicKeyHex));
+    });
+
+    test('one-wayness: neither key contains the seed, and HKDF is not '
+        'invertible', () {
+      final key = NostrKey.deriveFromSeed(seed);
+      final seedHex = seed
+          .map((b) => b.toRadixString(16).padLeft(2, '0'))
+          .join();
+      // The seed does not appear verbatim in the derived material.
+      expect(key.privateKeyHex, isNot(contains(seedHex)));
+      expect(key.publicKeyHex, isNot(contains(seedHex)));
+      // Structural argument (documented in findings §1): recovery of the seed
+      // would require inverting HMAC-SHA256. We assert the property we CAN test:
+      // two unrelated seeds never collide to the same key.
+      final other = NostrKey.deriveFromSeed(
+        Uint8List.fromList(List.generate(32, (i) => 255 - i)),
+      );
+      expect(key.privateKeyHex, isNot(other.privateKeyHex));
+    });
+
+    test('integrates with the real IdentityService seed length', () {
+      final rootSeed = IdentityService(
+        random: Random(42),
+      ).generateRootSeed();
+      expect(rootSeed.length, IdentityService.rootSeedLengthBytes);
+      final key = NostrKey.deriveFromSeed(rootSeed);
+      expect(key.publicKeyHex, matches(RegExp(r'^[0-9a-f]{64}$')));
+    });
+  });
+}