feat(core): support pure watch-only address import

Adds a WalletDocument.WatchAddresses field and WalletLoader.NewFromAddresses
to build an ImportedKeyAccount with no private key at all (unlike xpub/WIF
imports, which can still derive/hold key material). ScriptKind is inferred
from the address itself via a new DerivationPaths.KindFor helper. Signing
already refuses to run for any IsWatchOnly account (TransactionFactory), so
this only had to wire up construction/reload of the new account shape.

tests/PalladiumWallet.Tests/Storage/StorageTests.cs

@@ -148,6 +148,21 @@ public class StorageTests
         Assert.True(WalletDocument.FromJson(doc.ToJson()).IsWatchOnly);
     }
 
+    [Fact]
+    public void WatchAddresses_fa_roundtrip_json_ed_e_watch_only()
+    {
+        var doc = new WalletDocument
+        {
+            Network = "mainnet",
+            ScriptKind = "NativeSegwit",
+            WatchAddresses = ["bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kv8f3t4"],
+        };
+        var restored = WalletDocument.FromJson(doc.ToJson());
+
+        Assert.Equal(doc.WatchAddresses, restored.WatchAddresses);
+        Assert.True(restored.IsWatchOnly);
+    }
+
     [Fact]
     public void Json_corrotto_lancia_eccezione()
     {