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/nip99.dart

@@ -0,0 +1,118 @@
+import 'nostr_event.dart';
+import 'offer.dart';
+
+/// Maps [Offer] ↔ Nostr NIP-99 "Classified Listing" events (kind 30402), the
+/// first concrete `OfferTransport` backend (sharing-model.md §2).
+///
+/// This codec is where the privacy contract is *enforced on the wire*: the only
+/// location that ever leaves the device is a coarse geohash, truncated here to
+/// [maxGeohashChars]. There is no field that could carry the inventory or an
+/// exact address, because [Offer] itself has none — the seam does the work.
+class Nip99Codec {
+  /// NIP-99 addressable classified listing.
+  static const kindActive = 30402;
+
+  /// A geohash of 5 chars ≈ ±2.4 km cell — the coarse "near you" of the mocks.
+  /// Anything finer is refused so a slip in the caller can't leak a precise fix.
+  static const maxGeohashChars = 5;
+
+  /// Builds an UNSIGNED event from [offer]. Caller signs with the derived key.
+  NostrEvent encode(Offer offer, {required int createdAt}) {
+    final coarse = _coarsen(offer.approxGeohash);
+
+    final tags = <List<String>>[
+      ['d', offer.id],
+      ['title', offer.summary],
+      ['status', offer.status == OfferStatus.active ? 'active' : 'sold'],
+      // Reciprocity mode is a Tanemaki concept NIP-99 lacks; carry it explicitly.
+      ['offer_type', offer.type.name],
+    ];
+    // NIP-52 geohash ladder: one `g` tag per prefix length so an area query
+    // matches by *exact* tag (how Nostr relays filter) yet still finds coarser
+    // searches. Coarse only — NO "location"/address/exact-fix tag ever.
+    for (var i = 1; i <= coarse.length; i++) {
+      tags.add(['g', coarse.substring(0, i)]);
+    }
+    if (offer.category != null) tags.add(['t', offer.category!]);
+    if (offer.radiusKm != null) {
+      tags.add(['radius_km', '${offer.radiusKm}']);
+    }
+    if (offer.type == OfferType.sale && offer.priceAmount != null) {
+      tags.add([
+        'price',
+        '${offer.priceAmount}',
+        offer.priceCurrency ?? 'EUR',
+      ]);
+    }
+    if (offer.type == OfferType.exchange && offer.exchangeTerms != null) {
+      tags.add(['exchange_terms', offer.exchangeTerms!]);
+    }
+    if (offer.imageUrl != null) tags.add(['image', offer.imageUrl!]);
+    if (offer.expiresAt != null) {
+      tags.add([
+        'expiration',
+        '${offer.expiresAt!.millisecondsSinceEpoch ~/ 1000}',
+      ]);
+    }
+
+    return NostrEvent(
+      pubkey: offer.authorPubkeyHex,
+      createdAt: createdAt,
+      kind: kindActive,
+      tags: tags,
+      content: offer.summary,
+    );
+  }
+
+  /// Reconstructs an [Offer] from a received event.
+  Offer decode(NostrEvent event) {
+    final priceTag = _fullTag(event, 'price');
+    return Offer(
+      id: event.tag('d') ?? event.id,
+      authorPubkeyHex: event.pubkey,
+      summary: event.tag('title') ?? event.content,
+      type: _parseType(event.tag('offer_type')),
+      status: event.tag('status') == 'sold'
+          ? OfferStatus.closed
+          : OfferStatus.active,
+      category: event.tag('t'),
+      approxGeohash: _longestGeohash(event),
+      radiusKm: int.tryParse(event.tag('radius_km') ?? ''),
+      priceAmount: priceTag != null && priceTag.length > 1
+          ? num.tryParse(priceTag[1])
+          : null,
+      priceCurrency: priceTag != null && priceTag.length > 2
+          ? priceTag[2]
+          : null,
+      exchangeTerms: event.tag('exchange_terms'),
+      imageUrl: event.tag('image'),
+    );
+  }
+
+  String _coarsen(String geohash) => geohash.length > maxGeohashChars
+      ? geohash.substring(0, maxGeohashChars)
+      : geohash;
+
+  /// The finest `g` tag present (top of the prefix ladder).
+  String _longestGeohash(NostrEvent event) {
+    var best = '';
+    for (final t in event.tags) {
+      if (t.isNotEmpty && t[0] == 'g' && t.length > 1 && t[1].length > best.length) {
+        best = t[1];
+      }
+    }
+    return best;
+  }
+
+  List<String>? _fullTag(NostrEvent event, String name) {
+    for (final t in event.tags) {
+      if (t.isNotEmpty && t[0] == name) return t;
+    }
+    return null;
+  }
+
+  OfferType _parseType(String? name) => OfferType.values.firstWhere(
+    (t) => t.name == name,
+    orElse: () => OfferType.gift,
+  );
+}