feat(chat): message model, in-memory store, and receive service

Phase 1 Steps 4-6 (the receive pipeline, minus the App/sync wiring which
lands next).

- chat_message: the in-memory ChatMessage model (direction, kind, txid, cid,
  peer zaddr/pubkey, body, timestamp, payload_position). No libsodium.
- chat_store: threads messages by conversation_id and dedups by
  (txid, payload_position) so re-scanning the chain never double-inserts.
- chat_service: owns the long-lived chat identity keypair (move-disabled,
  wiped on destruction/clear) and the store. ingest() decrypts each Message
  (drops undecryptable ones silently — no plaintext/memo logging), passes a
  ContactRequest's plaintext through, and threads the result. No-op without an
  identity.

Tests: a metadata batch decrypts into a threaded conversation; re-ingest
dedups; a contact request carries through; a wrong identity decrypts nothing;
no-identity ingest is a no-op.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/chat/chat_store.cpp

@@ -0,0 +1,39 @@
+// DragonX Wallet - HushChat in-memory message store (implementation).
+
+#include "chat_store.h"
+
+namespace dragonx::chat {
+
+std::string ChatStore::dedupKey(const ChatMessage& message) {
+    return message.txid + ":" + std::to_string(message.payload_position);
+}
+
+bool ChatStore::append(const ChatMessage& message) {
+    if (!seen_.insert(dedupKey(message)).second) return false;
+    messages_.push_back(message);
+    return true;
+}
+
+std::vector<ChatMessage> ChatStore::conversation(const std::string& conversationId) const {
+    std::vector<ChatMessage> out;
+    for (const auto& message : messages_) {
+        if (message.conversation_id == conversationId) out.push_back(message);
+    }
+    return out;
+}
+
+std::vector<std::string> ChatStore::conversationIds() const {
+    std::vector<std::string> ids;
+    std::unordered_set<std::string> seenIds;
+    for (const auto& message : messages_) {
+        if (seenIds.insert(message.conversation_id).second) ids.push_back(message.conversation_id);
+    }
+    return ids;
+}
+
+void ChatStore::clear() {
+    messages_.clear();
+    seen_.clear();
+}
+
+} // namespace dragonx::chat