perf(history): scan shielded history in bounded passes, not every block

Every block re-ran z_listreceivedbyaddress for every z-address (O(wallet)
each, under cs_main) and a whole-wallet z_listsentbyaddress. On a 40-address
wallet that was ~80% of wall-clock. The history now advances in passes:

- an address is rescanned when it is stale by a number of blocks scaled to
  its measured cost (8 to 500), never-scanned and hinted addresses first
  (a hint: z_listunspent shows a note the history doesn't);
- z_listsentbyaddress covers only the blocks since the last good sweep
  (+100 reorg margin); the whole-wallet sweep runs once per connect, with
  a 10 min / 1 h / 6 h backoff after real failures;
- listtransactions pages are sized from last time, and an unchanged tip
  reuses the transparent rows;
- found-but-unenriched txids carry over as a backlog; only -5 is a
  definitive "not ours" for z_viewtransaction;
- between blocks, a cheap recent poll plus a one-address probe.

Duty model for a 40-address wallet at 10 s per scan: ~9% instead of ~80%.

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

src/app_security.cpp

@@ -547,7 +547,7 @@ void App::lockWallet() {
             if (ok) {
                 state_.locked = true;
                 state_.unlocked_until = 0;
-                resetTransactionHistoryCacheSession();
+                lockTransactionHistoryCacheSession();   // keeps the history's scan progress
                 lock_failure_warned_ = false;
                 DEBUG_LOGF("[App] Wallet locked\n");
             } else {
@@ -642,7 +642,7 @@ void App::refreshWalletEncryptionState() {
                         settings_->save();
                     }
                     if (state_.locked) {
-                        resetTransactionHistoryCacheSession();
+                        lockTransactionHistoryCacheSession();
                     } else if (state_.transactions.empty()) {
                         loadTransactionHistoryCacheIfAvailable();
                     } else {
@@ -1799,7 +1799,7 @@ void App::renderDecryptWalletDialog() {
                                                                     // Force address + peer refresh
                                                                     invalidateAddressValidationCache();
                                                                     addresses_dirty_ = true;
-                                                                    transactions_dirty_ = true;
+                                                                    markTransactionsDirty();
                                                                     last_tx_block_height_ = -1;
 
                                                                     refreshWalletEncryptionState();