perf(spv): persist checkpoint-anchoring state across sync sessions

_anchoredUpTo tracked which heights were already proven to hash-chain
back to a checkpoint, but only in memory: every app restart re-walked
and re-verified the whole header chain from the checkpoint even when
the header bytes themselves were already cached on disk, dominating
reconnect time on large wallets. Round-trip it through SyncCache like
the other sync caches (RawTxHex/VerifiedAt/BlockHeaders).

src/App/ViewModels/MainWindowViewModel.Sync.cs

@@ -271,7 +271,8 @@ public partial class MainWindowViewModel
                     _doc.Cache?.BlockHeaders,
                     _doc.Cache?.NextReceiveIndex ?? 0,
                     _doc.Cache?.NextChangeIndex ?? 0,
-                    net);
+                    net,
+                    _doc.Cache?.AnchoredUpTo);
                 _synchronizer.Progress += msg => Dispatcher.UIThread.Post(() => StatusMessage = msg);
                 // Progressive verification (§7.4): the wallet becomes usable as soon as
                 // transaction downloads finish, without waiting for every historical Merkle
@@ -289,15 +290,19 @@ public partial class MainWindowViewModel
                 // Off the UI thread: sync does heavy CPU work (LINQ over thousands of
                 // cached txs/UTXOs) and blocking JSON/disk I/O between awaits, negligible
                 // on desktop but enough to freeze the UI (ANR) on slower mobile hardware.
-                var (result, rawHex, verifiedAt, blockHeaders) = await Task.Run(async () =>
+                var (result, rawHex, verifiedAt, blockHeaders, anchoredUpTo) = await Task.Run(async () =>
                 {
                     var r = await _synchronizer.SyncOnceAsync(ct);
                     var caches = _synchronizer.ExportCaches(PalladiumNetworks.For(_account.Profile.Kind));
-                    return (r, caches.RawTxHex, caches.VerifiedAt, caches.BlockHeaders);
+                    return (r, caches.RawTxHex, caches.VerifiedAt, caches.BlockHeaders, caches.AnchoredUpTo);
                 }, ct);
                 _lastTransactions = result.Transactions;
 
-                var cache = new SyncCache { RawTxHex = rawHex, VerifiedAt = verifiedAt, BlockHeaders = blockHeaders };
+                var cache = new SyncCache
+                {
+                    RawTxHex = rawHex, VerifiedAt = verifiedAt, BlockHeaders = blockHeaders,
+                    AnchoredUpTo = anchoredUpTo,
+                };
                 FillDisplayFields(cache, result);
                 _doc.Cache = cache;
                 await WalletStore.SaveAsync(_doc, _walletPath!, _password);
@@ -493,12 +498,13 @@ public partial class MainWindowViewModel
         try
         {
             var net = PalladiumNetworks.For(_account.Profile.Kind);
-            var (rawHex, verifiedAt, blockHeaders) = _synchronizer.ExportCaches(net);
+            var (rawHex, verifiedAt, blockHeaders, anchoredUpTo) = _synchronizer.ExportCaches(net);
             if (rawHex.Count == 0 && verifiedAt.Count == 0)
                 return;
             (_doc.Cache ??= new SyncCache()).RawTxHex = rawHex;
             _doc.Cache.VerifiedAt = verifiedAt;
             _doc.Cache.BlockHeaders = blockHeaders;
+            _doc.Cache.AnchoredUpTo = anchoredUpTo;
             WalletStore.Save(_doc, _walletPath, _password);
         }
         catch { /* non-fatal: the next full save will recover */ }