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/Crypto/AddressDerivationTests.cs

@@ -20,7 +20,7 @@ public class AddressDerivationTests
         // Known abandon-about address at m/44'/0'/0'/0/0 (public reference).
         var account = HdAccount.FromSeed(AbandonAboutSeed(), ScriptKind.Legacy,
             ChainProfiles.Mainnet, new KeyPath("44'/0'/0'"));
-        var pubKey = account.GetPublicKey(isChange: false, 0);
+        var pubKey = account.GetPublicKey(isChange: false, 0)!;
 
         var bitcoinAddr = pubKey.GetAddress(ScriptPubKeyType.Legacy, Network.Main);
         Assert.Equal("1LqBGSKuX5yYUonjxT5qGfpUsXKYYWeabA", bitcoinAddr.ToString());
@@ -37,7 +37,7 @@ public class AddressDerivationTests
     {
         var account = HdAccount.FromSeed(AbandonAboutSeed(), ScriptKind.WrappedSegwit,
             ChainProfiles.Mainnet, new KeyPath("49'/0'/0'"));
-        var pubKey = account.GetPublicKey(isChange: false, 0);
+        var pubKey = account.GetPublicKey(isChange: false, 0)!;
 
         var bitcoinAddr = pubKey.GetAddress(ScriptPubKeyType.SegwitP2SH, Network.Main);
         Assert.Equal("37VucYSaXLCAsxYyAPfbSi9eh4iEcbShgf", bitcoinAddr.ToString());