feat: track shielded send txids via z_viewtransaction

Extract txids from completed z_sendmany operations and store in
send_txids_ so pure shielded sends are discoverable. The network
thread includes them in the enrichment set, calls z_viewtransaction,
caches results in viewtx_cache_, and removes them from send_txids_.

src/app.h

@@ -494,6 +494,12 @@ private:
     float opid_poll_timer_ = 0.0f;
     static constexpr float OPID_POLL_INTERVAL = 2.0f;
 
+    // Txids from completed z_sendmany operations.
+    // Ensures shielded sends are discoverable by z_viewtransaction
+    // even when they don't appear in listtransactions or
+    // z_listreceivedbyaddress.
+    std::unordered_set<std::string> send_txids_;
+
     // First-run wizard state
     WizardPhase wizard_phase_ = WizardPhase::None;
     std::unique_ptr<util::Bootstrap> bootstrap_;