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).