test: extend wallet/storage/property coverage and fix nullable warnings

- AppPaths: data-root resolution, per-network paths, wallet file listing.
- TransactionFactory: legacy/P2SH/segwit destinations, multi-UTXO selection,
  dust change absorbed into fee, and a golden txid for the PSBT signing path
  (RFC 6979 makes it deterministic; the builder's own output shuffling for
  privacy makes anchoring the vector at the top-level Build() unreliable).
- PropertyTests: Slip132 roundtrip for every script kind and network,
  WalletDocument JSON roundtrip with arbitrary labels/contacts, Scripthash
  cross-checked against an independent SHA-256 computation.
- StorageTests: regression cases for the IsEncrypted fix.
- Removed CS8602/CS8604 nullable warnings from existing address/loader tests.

tests/PalladiumWallet.Tests/Wallet/WalletLoaderTests.cs

@@ -28,8 +28,8 @@ public class WalletLoaderTests
         Assert.Equal("NativeSegwit", doc.ScriptKind);
         Assert.Equal(ValidMnemonic, doc.Mnemonic);
         Assert.Null(doc.Passphrase);
-        Assert.NotEmpty(doc.AccountXpub);
-        Assert.NotEmpty(doc.MasterFingerprint);
+        Assert.NotEmpty(doc.AccountXpub!);
+        Assert.NotEmpty(doc.MasterFingerprint!);
         Assert.False(doc.IsWatchOnly);
     }
 
@@ -66,7 +66,7 @@ public class WalletLoaderTests
         var (doc, _) = WalletLoader.NewFromMnemonic(
             ValidMnemonic24, null, ScriptKind.NativeSegwit, ChainProfiles.Mainnet);
         Assert.Equal("mainnet", doc.Network);
-        Assert.NotEmpty(doc.AccountXpub);
+        Assert.NotEmpty(doc.AccountXpub!);
     }
 
     [Fact]