fix(spv): anchor headers up to a checkpoint above them

With proof of work skipped, chaining headers from a checkpoint up to a
transaction's block proves nothing: a server can fabricate a header and
fake headers after a real checkpoint. The header must now hash-chain up to
the nearest checkpoint at or above its height, whose hash is hardcoded.
This also anchors heights below the first checkpoint. The cached anchor
state is renamed AnchoredDownTo and caches without it are re-verified
once. Blocks above the newest checkpoint remain a documented limit.
See ADR 0040.

src/App/ViewModels/MainWindowViewModel.Sync.cs

@@ -281,7 +281,7 @@ public partial class MainWindowViewModel
                     _doc.Cache?.NextReceiveIndex ?? 0,
                     _doc.Cache?.NextChangeIndex ?? 0,
                     net,
-                    _doc.Cache?.AnchoredUpTo);
+                    _doc.Cache?.AnchoredDownTo);
                 _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
@@ -299,18 +299,18 @@ 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, anchoredUpTo) = await Task.Run(async () =>
+                var (result, rawHex, verifiedAt, blockHeaders, anchoredDownTo) = 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, caches.AnchoredUpTo);
+                    return (r, caches.RawTxHex, caches.VerifiedAt, caches.BlockHeaders, caches.AnchoredDownTo);
                 }, ct);
                 _lastTransactions = result.Transactions;
 
                 var cache = new SyncCache
                 {
                     RawTxHex = rawHex, VerifiedAt = verifiedAt, BlockHeaders = blockHeaders,
-                    AnchoredUpTo = anchoredUpTo,
+                    AnchoredDownTo = anchoredDownTo,
                 };
                 FillDisplayFields(cache, result);
                 _doc.Cache = cache;
@@ -532,13 +532,13 @@ public partial class MainWindowViewModel
         try
         {
             var net = PalladiumNetworks.For(_account.Profile.Kind);
-            var (rawHex, verifiedAt, blockHeaders, anchoredUpTo) = _synchronizer.ExportCaches(net);
+            var (rawHex, verifiedAt, blockHeaders, anchoredDownTo) = _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;
+            _doc.Cache.AnchoredDownTo = anchoredDownTo;
             WalletStore.Save(_doc, _walletPath, _password);
         }
         catch { /* non-fatal: the next full save will recover */ }