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/Bip84Slip132Tests.cs

@@ -83,7 +83,7 @@ public class Bip84Slip132Tests
         bool isChange, int index, string? expectedPubKeyHex, string bitcoinAddress)
     {
         var account = Account();
-        var pubKey = account.GetPublicKey(isChange, index);
+        var pubKey = account.GetPublicKey(isChange, index)!;
 
         if (expectedPubKeyHex is not null)
             Assert.Equal(expectedPubKeyHex, pubKey.ToHex());