feat(chat): wire the two-variant send transport (Phase 5)

Make composed messages actually reach the wire: broadcastChatMemos sends the
header + payload as two 0-value memo outputs to the peer's z-address (header
first, the lower memo position), on both variants.

- chat_outgoing: chatSendOutputs(memos, utf8Prefix) — the pure, testable memo
  encoder. Full-node memos get a "utf8:" prefix (dragonxd rejects raw JSON and
  then UTF-8-encodes on-chain, byte-identical to SilentDragonXLite's
  Memo::from_str); the payload is NOT double-hex. Lite memos are raw UTF-8 (the
  backend does Memo::from_str directly). Header is always output 0.
- App::broadcastChatMemos: full-node builds a two-recipient z_sendmany array
  (amount 0, from the spendable reply z-address, default fee) via submitZSendMany
  with markFeeGapRetry=true — deliberately, to suppress the fee-gap auto-retry,
  which rebuilds a single-recipient tx and would drop the payload output. Lite
  routes to broadcastChatMemosLite (two 0-value LiteSendRecipients, raw memos;
  the backend accepts duplicate addresses + 0-value for exactly this pattern).
- Encoding + design established by a four-codebase mapping (wallet, daemon, SDXL,
  lite backend) and an adversarial review of the wiring (0 confirmed findings).
- Tests: chatSendOutputs (utf8:-prefix + header-first for full-node, raw for
  lite) + on-chain round-trip (strip "utf8:" -> the harvest parser re-pairs and
  decrypts).

Remaining as a LIVE test (cannot be proven from source): that dragonxd returns
the memo under memoStr verbatim, that recipient-array order maps to note
position, that a 0-value memo-only tx relays, and full SDXL<->DragonX interop.

Gated by DRAGONX_ENABLE_CHAT (default OFF). Verified: Linux + Windows(mingw)
build with chat ON, ctest 100%, hygiene clean; caches restored to the OFF default.

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

src/app.h

@@ -170,9 +170,8 @@ public:
     wallet::LiteWalletController* liteWallet() { return lite_wallet_.get(); }
     // HushChat service (identity + in-memory message store); the Chat tab reads its store.
     chat::ChatService& chatService() { return chat_service_; }
-    // HushChat composing (Phase 4): construct + locally echo an outgoing message (to a conversation
-    // whose peer key we know) / a new-conversation contact request. Network delivery is wired in a
-    // later phase — see broadcastChatMemos.
+    // HushChat composing: construct, broadcast (broadcastChatMemos), and locally echo an outgoing
+    // message (to a conversation whose peer key we know) / a new-conversation contact request.
     void sendChatMessage(const std::string& conversationId, const std::string& text);
     void startChatConversation(const std::string& peerZaddr, const std::string& text);
     // Reason the lite wallet failed to auto-open this session (empty if none / opened OK).
@@ -596,7 +595,8 @@ private:
     void provisionChatIdentityFromSecret(std::string secret);
     std::string chatReplyZaddr() const;                          // a stable wallet z-addr for outgoing headers
     std::string generateChatLocalId(const char* prefix, int numBytes) const;  // unique echo id / cid
-    void broadcastChatMemos(const chat::OutgoingChatMemos& memos);  // Phase-5 seam (network delivery)
+    void broadcastChatMemos(const chat::OutgoingChatMemos& memos);      // full-node z_sendmany / lite
+    void broadcastChatMemosLite(const chat::OutgoingChatMemos& memos);  // lite two-recipient send
     // Lite first-run welcome prompt: dismissed for the session once the user picks an action.
     bool lite_firstrun_dismissed_ = false;
     // Lite send-time unlock: set to show the unlock modal when a spend is attempted while locked.