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/Cli/Program.cs

@@ -140,10 +140,11 @@ static async Task<int> Sync(string[] o)
         doc.Cache?.BlockHeaders,
         doc.Cache?.NextReceiveIndex ?? 0,
         doc.Cache?.NextChangeIndex ?? 0,
-        net);
+        net,
+        doc.Cache?.AnchoredUpTo);
     var result = await sync.SyncOnceAsync();
 
-    var (rawHex, verifiedAt, blockHeaders) = sync.ExportCaches(net);
+    var (rawHex, verifiedAt, blockHeaders, anchoredUpTo) = sync.ExportCaches(net);
     doc.Cache = new SyncCache
     {
         TipHeight = result.TipHeight,
@@ -160,6 +161,7 @@ static async Task<int> Sync(string[] o)
         RawTxHex = rawHex,
         VerifiedAt = verifiedAt,
         BlockHeaders = blockHeaders,
+        AnchoredUpTo = anchoredUpTo,
     };
     WalletStore.Save(doc, path, Opt(o, "--password"));