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/chat/chat_outgoing.cpp

@@ -33,6 +33,15 @@ bool present(const std::string& value) { return !value.empty(); }
 
 } // namespace
 
+std::array<ChatSendOutput, 2> chatSendOutputs(const OutgoingChatMemos& memos, bool utf8Prefix)
+{
+    const std::string prefix = utf8Prefix ? "utf8:" : "";
+    return {{
+        { memos.recipientZaddr, prefix + memos.headerMemo },   // header — the lower memo position
+        { memos.recipientZaddr, prefix + memos.payloadMemo },
+    }};
+}
+
 ChatComposeStatus buildOutgoingMessage(const ChatKeyPair& mine,
                                        const std::string& myPublicKeyHex,
                                        const std::string& myReplyZaddr,