perf(chat): sends re-read by origin, chat scans under the scan budget

A z_sendmany submit set off a burst of wallet scans before anything had
changed. Ops now re-read the wallet when they finish, by who started them:
the user's sends and dialog ops get a re-read plus one forced balance once
none of their ops is pending; client auto-shield re-reads unforced; chat
messages and splits refresh only the chat note view. A pending send no
longer forces scans, and chat txids no longer enter the tracked-send set.

Chat ingest skips stored messages before decrypting and caches
undecryptable ones per identity. The inbox scan runs when getwalletinfo's
txcount changed (a cheap probe, at most 2% duty) or a cost-scaled safety
net is due; the note-buffer scan runs on need. Both are admitted by the
scan budget, on a steady clock. The chat payer comes from a per-address
view of verified notes, with debits re-applied across in-flight scans;
splits are sized to one payer and only when they can refill the buffer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/chat/chat_service.h

@@ -14,6 +14,7 @@
 #include <functional>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 namespace dragonx::chat {
@@ -99,9 +100,15 @@ public:
     const ChatStore& store() const { return store_; }
     ChatStore& store() { return store_; }
 
+    std::uint64_t decryptAttempts() const { return decrypt_attempts_; }   // tests / diagnostics
+
 private:
     ChatKeyPair identity_{};
     bool has_identity_ = false;
+    // Decrypt failures, keyed by what the decrypt reads (bounded). A rescan re-sends every message ever
+    // received, so without it each poll re-ran the crypto for every undecryptable memo.
+    std::unordered_set<std::string> undecryptable_;
+    std::uint64_t decrypt_attempts_ = 0;
     ChatStore store_;
     ChatDatabase* db_ = nullptr;  // optional; not owned
     std::function<bool(const std::string&)> blocked_pred_;  // true => cid is blocked (drop its messages)