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.h

@@ -0,0 +1,37 @@
+#pragma once
+
+// DragonX Wallet - HushChat in-memory message store (Phase 1; sqlite persistence is Phase 2).
+
+#include "chat_message.h"
+
+#include <string>
+#include <unordered_set>
+#include <vector>
+
+namespace dragonx::chat {
+
+// Threads messages by conversation_id (cid) and deduplicates by (txid, payload_position) so
+// re-scanning the chain never double-inserts. Not thread-safe — drive from the main thread.
+class ChatStore {
+public:
+    // Returns true if newly inserted, false if a duplicate was ignored.
+    bool append(const ChatMessage& message);
+
+    // Messages in a conversation, in insertion order.
+    std::vector<ChatMessage> conversation(const std::string& conversationId) const;
+
+    // Distinct conversation ids, in first-seen order.
+    std::vector<std::string> conversationIds() const;
+
+    std::size_t size() const { return messages_.size(); }
+    bool empty() const { return messages_.empty(); }
+    void clear();
+
+private:
+    static std::string dedupKey(const ChatMessage& message);
+
+    std::vector<ChatMessage> messages_;
+    std::unordered_set<std::string> seen_;
+};
+
+} // namespace dragonx::chat