feat(block2): offers logic layer — OffersCubit + OfferMapper (no UI yet)

The testable core of the offers UI slice, transport-agnostic:
- OfferMapper: pure app-sharing-intent (OfferStatus) -> network Offer/OfferType
  mapping; keeps price only for sales, refuses to publish private lots.
- OffersCubit: discover(geohash) streams results; publish() reports the verdict;
  depends on the OfferTransport interface (fake in tests) and degrades
  gracefully offline (null transport = social layer unavailable, never throws).

8 tests green (fake in-memory transport, no relay). Screen/routing/i18n come
next — they need two product decisions (coarse-area capture, relay list).

apps/app_seeds/lib/services/offer_mapper.dart

@@ -0,0 +1,53 @@
+import 'package:commons_core/commons_core.dart';
+
+import '../db/enums.dart';
+
+/// Turns a locally-shared lot into a publishable [Offer]. Pure — no I/O — so it
+/// is easy to test and keeps the privacy seam explicit: only the fields passed
+/// here ever reach the network (never the whole lot/inventory, never an exact
+/// address; the area is a coarse geohash chosen by the user).
+class OfferMapper {
+  const OfferMapper._();
+
+  /// Maps the app's local sharing intent ([OfferStatus]) to the network offer
+  /// type. `private` lots are never published.
+  static OfferType offerTypeFor(OfferStatus sharing) => switch (sharing) {
+        OfferStatus.shared => OfferType.gift,
+        OfferStatus.exchange => OfferType.exchange,
+        OfferStatus.sell => OfferType.sale,
+        OfferStatus.private =>
+          throw ArgumentError('private lots are not published to the network'),
+      };
+
+  /// Builds the [Offer] for a shared lot. [lotId] gives the offer a stable,
+  /// addressable id (re-publishing updates in place). [areaGeohash] is the
+  /// user's coarse area — the transport coarsens it further on the wire.
+  static Offer fromSharedLot({
+    required String lotId,
+    required String authorPubkeyHex,
+    required String summary,
+    required OfferStatus sharing,
+    required String areaGeohash,
+    String? category,
+    num? priceAmount,
+    String? priceCurrency,
+    String? exchangeTerms,
+    String? imageUrl,
+    DateTime? expiresAt,
+  }) {
+    final type = offerTypeFor(sharing);
+    return Offer(
+      id: lotId,
+      authorPubkeyHex: authorPubkeyHex,
+      summary: summary,
+      type: type,
+      approxGeohash: areaGeohash,
+      category: category,
+      priceAmount: type == OfferType.sale ? priceAmount : null,
+      priceCurrency: type == OfferType.sale ? priceCurrency : null,
+      exchangeTerms: type == OfferType.exchange ? exchangeTerms : null,
+      imageUrl: imageUrl,
+      expiresAt: expiresAt,
+    );
+  }
+}