perf(spv): incremental discovery, parallel chains, persistent header cache

Speed up wallet synchronization, especially re-syncs of large wallets:

- Incremental address discovery: known indices from the previous sync
  (NextReceiveIndex/NextChangeIndex) are fetched in a single parallel
  burst instead of sequential gap-limit rounds, cutting round-trips from
  O(used/gapLimit) to ~O(1). Receive and change chains scan in parallel.
- Merge tx download and Merkle verification into one parallel phase
  (no barrier between them); the per-call semaphore is dropped in favor
  of the transport-level in-flight gate.
- Persist verified block headers (SyncCache.BlockHeaders, height -> hex):
  immutable once stored, so blockchain.block.header round-trips for
  already-verified proofs are eliminated on later syncs.
- RetryOnBusyAsync with exponential backoff around server calls, treating
  -101/-102 and "server busy"/"excessive resource usage" as transient.

Wire the new PreloadCaches/ExportCaches signatures through the desktop
view model and the CLI sync command.

src/Core/Storage/WalletDocument.cs

@@ -113,6 +113,13 @@ public sealed class SyncCache
     /// riverificare le stesse prove ad ogni avvio: le conferme sono immutabili.
     /// </summary>
     public Dictionary<string, int>? VerifiedAt { get; set; }
+
+    /// <summary>
+    /// Header grezzi per altezza (altezza → hex). Immutabili: non vengono mai
+    /// rifetchati una volta salvati. Elimina i GetBlockHeader sulle prove Merkle
+    /// già verificate nei sync successivi.
+    /// </summary>
+    public Dictionary<int, string>? BlockHeaders { get; set; }
 }
 
 /// <summary>Indirizzo scansionato con saldo proprio e numero di transazioni (vista indirizzi).</summary>