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/Core/Storage/WalletDocument.cs

@@ -134,6 +134,14 @@ public sealed class SyncCache
     /// subsequent syncs.
     /// </summary>
     public Dictionary<int, string>? BlockHeaders { get; set; }
+
+    /// <summary>
+    /// Checkpoint height → highest height already proven to hash-chain back to it (§7.3).
+    /// Avoids re-walking and re-verifying the whole header chain from the checkpoint on
+    /// every launch: the chain-of-hashes check already done for a height doesn't need
+    /// redoing once headers themselves are cached.
+    /// </summary>
+    public Dictionary<int, int>? AnchoredUpTo { get; set; }
 }
 
 /// <summary>Scanned address with its own balance and transaction count (address view).</summary>