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/offer_transport.dart

@@ -0,0 +1,29 @@
+import 'offer.dart';
+
+/// The abstraction the plan promises lives in `commons_core` (core-domain-
+/// boundary.md §2, PLAN §4): publish an [Offer], discover offers by area. The
+/// concrete Nostr NIP-99 backend sits behind this so the domain never talks to
+/// a relay directly, and a second backend (ActivityPub / FEP-0837) could slot
+/// in without the app noticing.
+///
+/// Spike Question 2 asks whether messaging (NIP-17) and trust (NIP-85) fit
+/// behind this same seam, or whether they couple more than the plan implies.
+/// The finding: they do NOT belong here — they share the *key* and the *relay
+/// connection*, but their verbs differ (send/receive a private DM; assert/read
+/// a certification). Forcing them behind `OfferTransport` would overload it.
+/// See spike-block2-findings.md §2 for the proposed 3-interface split over one
+/// shared `NostrConnection`.
+abstract interface class OfferTransport {
+  /// Publishes (or replaces) [offer]. Idempotent on the offer id.
+  Future<PublishResult> publish(Offer offer);
+
+  /// Streams offers matching [query]. Emits already-stored matches, then live
+  /// ones as they arrive, until the caller cancels the subscription.
+  Stream<Offer> discover(DiscoveryQuery query);
+
+  /// Retracts a previously published offer (NIP-99: publish a `closed` status /
+  /// NIP-09 deletion). Relays that already replicated it drop it over time.
+  Future<void> retract(String offerId);
+
+  Future<void> close();
+}