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.

src/Core/Crypto/DerivationPaths.cs

@@ -52,6 +52,19 @@ public static class DerivationPaths
         _ => throw new ArgumentOutOfRangeException(nameof(kind)),
     };
 
+    /// <summary>
+    /// Best-effort reverse mapping from an already-known address to a ScriptKind, used to
+    /// label pure address imports (no derivation involved, so this is informational only).
+    /// </summary>
+    public static ScriptKind KindFor(BitcoinAddress address) => address switch
+    {
+        BitcoinWitPubKeyAddress => ScriptKind.NativeSegwit,
+        TaprootAddress => ScriptKind.Taproot,
+        BitcoinWitScriptAddress => ScriptKind.NativeSegwitMultisig,
+        BitcoinScriptAddress => ScriptKind.WrappedSegwit,
+        _ => ScriptKind.Legacy,
+    };
+
     /// <summary>
     /// Account path relative to the root: purpose'/coin'/account' (§4.2).
     /// coin_type is taken from the profile (746 mainnet, 1 testnet).

src/Core/Storage/WalletDocument.cs

@@ -40,6 +40,9 @@ public sealed class WalletDocument
     /// <summary>Imported WIF keys (in plaintext in the document — must be encrypted!).</summary>
     public List<string>? WifKeys { get; set; }
 
+    /// <summary>Watch-only addresses with no associated private key (pure address import).</summary>
+    public List<string>? WatchAddresses { get; set; }
+
     /// <summary>Gap limit for address scanning (§5), configurable.</summary>
     public int GapLimit { get; set; } = 20;
 

src/Core/Wallet/WalletLoader.cs

@@ -51,7 +51,15 @@ public static class WalletLoader
             return new ImportedKeyAccount(entries, kind, profile);
         }
 
-        // 4. Watch-only from xpub
+        // 4. Watch-only imported addresses (no keys, no HD derivation)
+        if (doc.WatchAddresses is { Count: > 0 } watchAddresses)
+        {
+            var entries = watchAddresses.Select(a =>
+                (BitcoinAddress.Create(a.Trim(), network), (Key?)null)).ToList();
+            return new ImportedKeyAccount(entries, kind, profile);
+        }
+
+        // 5. Watch-only from xpub
         if (doc.AccountXpub is null)
             throw new InvalidDataException("Wallet file has no xpub and no seed.");
         if (!Slip132.TryDecodePublic(doc.AccountXpub, profile, out var xpub, out _))
@@ -170,4 +178,47 @@ public static class WalletLoader
         };
         return (doc, account);
     }
+
+    /// <summary>
+    /// Creates the document from one or more plain addresses, with no private key at all
+    /// (pure watch-only import — the account can never sign, unlike xpub- or WIF-based accounts
+    /// which can be upgraded later by supplying the matching key). ScriptKind is informational
+    /// only here (no derivation happens from a fixed address list) and is auto-detected from the
+    /// first address unless <paramref name="kindOverride"/> is given.
+    /// </summary>
+    public static (WalletDocument Doc, ImportedKeyAccount Account) NewFromAddresses(
+        IReadOnlyList<string> addresses, ChainProfile profile, ScriptKind? kindOverride = null)
+    {
+        if (addresses.Count == 0)
+            throw new InvalidDataException("At least one address is required.");
+
+        var network = PalladiumNetworks.For(profile.Kind);
+        var entries = new List<(BitcoinAddress, Key?)>();
+        var addressStrings = new List<string>();
+
+        foreach (var raw in addresses)
+        {
+            BitcoinAddress addr;
+            try
+            {
+                addr = BitcoinAddress.Create(raw.Trim(), network);
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidDataException($"Invalid address: {ex.Message}");
+            }
+            entries.Add((addr, null));
+            addressStrings.Add(raw.Trim());
+        }
+
+        var kind = kindOverride ?? DerivationPaths.KindFor(entries[0].Item1);
+        var account = new ImportedKeyAccount(entries, kind, profile);
+        var doc = new WalletDocument
+        {
+            Network = profile.NetName,
+            ScriptKind = kind.ToString(),
+            WatchAddresses = addressStrings,
+        };
+        return (doc, account);
+    }
 }

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()
     {

tests/PalladiumWallet.Tests/Wallet/TransactionFactoryTests.cs

@@ -250,6 +250,33 @@ public class TransactionFactoryTests
         Assert.Contains(tx.Outputs, o => o.Value.Satoshi == 400_000);
     }
 
+    [Fact]
+    public void Un_account_di_soli_indirizzi_watch_only_produce_una_psbt_non_firmata()
+    {
+        var full = Account();
+        var (utxos, txs) = Fund(full, 1_000_000);
+
+        // Pure address import: no private key at all (unlike xpub watch-only, which
+        // can still derive public keys/scriptPubKeys — this account can't upgrade
+        // to spendable without the user separately importing the matching key).
+        var watchOnly = new ImportedKeyAccount(
+            [(full.GetReceiveAddress(0), (Key?)null)], ScriptKind.NativeSegwit, Profile);
+
+        var built = new TransactionFactory(watchOnly).Build(
+            utxos, txs, full.GetReceiveAddress(5), amountSats: 400_000,
+            feeRateSatPerVByte: 2, changeIndex: 0, tipHeight: 100);
+
+        Assert.False(built.Signed);
+        Assert.True(watchOnly.IsWatchOnly);
+        Assert.Null(watchOnly.GetPrivateKey(false, 0));
+
+        var psbt = built.Psbt;
+        psbt.SignWithKeys(full.GetExtPrivateKey(false, 0));
+        psbt.Finalize();
+        var tx = psbt.ExtractTransaction();
+        Assert.Contains(tx.Outputs, o => o.Value.Satoshi == 400_000);
+    }
+
     [Theory]
     [InlineData("0.00000001", 1L)]
     [InlineData("1", 100_000_000L)]

tests/PalladiumWallet.Tests/Wallet/WalletLoaderTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using PalladiumWallet.Core.Chain;
 using PalladiumWallet.Core.Crypto;
 using PalladiumWallet.Core.Storage;
@@ -249,4 +250,85 @@ public class WalletLoaderTests
         Assert.Throws<InvalidDataException>(
             () => WalletLoader.NewFromWif([], ScriptKind.NativeSegwit, ChainProfiles.Mainnet));
     }
+
+    // ---- NewFromAddresses (pure watch-only, no key at all) ----
+
+    private static string SampleAddress(ScriptKind kind = ScriptKind.NativeSegwit) =>
+        WalletLoader.NewFromMnemonic(ValidMnemonic, null, kind, ChainProfiles.Mainnet)
+            .Account.GetReceiveAddress(0).ToString();
+
+    [Fact]
+    public void NewFromAddresses_crea_documento_watch_only_senza_chiavi()
+    {
+        var address = SampleAddress();
+        var (doc, account) = WalletLoader.NewFromAddresses([address], ChainProfiles.Mainnet);
+
+        Assert.Equal("mainnet", doc.Network);
+        Assert.Null(doc.Mnemonic);
+        Assert.Null(doc.AccountXprv);
+        Assert.Null(doc.AccountXpub);
+        Assert.Null(doc.WifKeys);
+        Assert.Equal([address], doc.WatchAddresses);
+        Assert.True(doc.IsWatchOnly);
+        Assert.True(account.IsWatchOnly);
+        Assert.Null(account.GetPrivateKey(false, 0));
+        Assert.Equal(address, account.GetReceiveAddress(0).ToString());
+    }
+
+    [Fact]
+    public void NewFromAddresses_rileva_lo_scriptkind_dallindirizzo()
+    {
+        var legacyAddr = SampleAddress(ScriptKind.Legacy);
+        var (doc, _) = WalletLoader.NewFromAddresses([legacyAddr], ChainProfiles.Mainnet);
+        Assert.Equal("Legacy", doc.ScriptKind);
+    }
+
+    [Fact]
+    public void NewFromAddresses_piu_indirizzi_sono_tutti_scansionabili()
+    {
+        var addr1 = SampleAddress();
+        var addr2 = WalletLoader.NewFromMnemonic(ValidMnemonic24, null, ScriptKind.NativeSegwit, ChainProfiles.Mainnet)
+            .Account.GetReceiveAddress(0).ToString();
+        var (_, account) = WalletLoader.NewFromAddresses([addr1, addr2], ChainProfiles.Mainnet);
+
+        Assert.NotNull(account.FixedAddresses);
+        Assert.Equal(2, account.FixedAddresses!.Count);
+        Assert.True(account.FixedAddresses!.All(e => account.GetPrivateKey(e.IsChange, e.Index) is null));
+    }
+
+    [Fact]
+    public void NewFromAddresses_lista_vuota_lancia_eccezione()
+    {
+        Assert.Throws<InvalidDataException>(
+            () => WalletLoader.NewFromAddresses([], ChainProfiles.Mainnet));
+    }
+
+    [Fact]
+    public void NewFromAddresses_indirizzo_invalido_lancia_eccezione()
+    {
+        Assert.Throws<InvalidDataException>(
+            () => WalletLoader.NewFromAddresses(["not-an-address"], ChainProfiles.Mainnet));
+    }
+
+    [Fact]
+    public void NewFromAddresses_indirizzo_di_rete_sbagliata_lancia_eccezione()
+    {
+        var testnetAddr = WalletLoader.NewFromMnemonic(ValidMnemonic, null, ScriptKind.NativeSegwit, ChainProfiles.Testnet)
+            .Account.GetReceiveAddress(0).ToString();
+        Assert.Throws<InvalidDataException>(
+            () => WalletLoader.NewFromAddresses([testnetAddr], ChainProfiles.Mainnet));
+    }
+
+    [Fact]
+    public void ToAccount_da_watch_addresses_ricostruisce_lo_stesso_account()
+    {
+        var address = SampleAddress();
+        var (doc, _) = WalletLoader.NewFromAddresses([address], ChainProfiles.Mainnet);
+
+        var account = WalletLoader.ToAccount(doc);
+
+        Assert.True(account.IsWatchOnly);
+        Assert.Equal(address, account.GetReceiveAddress(0).ToString());
+        Assert.Null(account.GetPrivateKey(false, 0));
+    }
 }