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_store.h

@@ -20,6 +20,13 @@ public:
     // Returns true if newly inserted, false if a duplicate was ignored.
     bool append(const ChatMessage& message);
 
+    // The dedup key of a message: "<txid>:<payload_position>".
+    static std::string messageKey(const std::string& txid, std::size_t payloadPosition);
+    // A message with this (txid, position) is in the store (append would refuse it).
+    bool contains(const std::string& txid, std::size_t payloadPosition) const {
+        return seen_.count(messageKey(txid, payloadPosition)) != 0;
+    }
+
     // Messages in a conversation, in insertion order.
     std::vector<ChatMessage> conversation(const std::string& conversationId) const;