test(core): cover error paths, boundaries and concurrency

Empty header range during checkpoint anchoring, send-all limits, SyncCache
roundtrip, corrupted config, derivation edge cases, spendability
boundaries, unit formatting, loader errors, in-flight cap, disposal under
load and wallet-lock contention. Also raises the synchronizer partial-result
timeouts so they hold under load.

tests/PalladiumWallet.Tests/Crypto/DerivationPathsEdgeCaseTests.cs

@@ -0,0 +1,56 @@
+using NBitcoin;
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Crypto;
+
+namespace PalladiumWallet.Tests.Crypto;
+
+public class DerivationPathsEdgeCaseTests
+{
+    [Theory]
+    [InlineData(ScriptKind.WrappedSegwitMultisig)]
+    [InlineData(ScriptKind.NativeSegwitMultisig)]
+    public void Multisig_Kinds_FailLoudlyInsteadOfDerivingAWrongWallet(ScriptKind kind) =>
+        Assert.Throws<NotSupportedException>(() => DerivationPaths.ScriptPubKeyTypeFor(kind));
+
+    [Fact]
+    public void Multisig_UsesBip48Purpose_AndTaprootBip86()
+    {
+        Assert.Equal(48, DerivationPaths.PurposeFor(ScriptKind.NativeSegwitMultisig));
+        Assert.Equal(48, DerivationPaths.PurposeFor(ScriptKind.WrappedSegwitMultisig));
+        Assert.Equal(86, DerivationPaths.PurposeFor(ScriptKind.Taproot));
+    }
+
+    [Fact]
+    public void UnknownScriptKind_IsRejected()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => DerivationPaths.PurposeFor((ScriptKind)999));
+        Assert.Throws<ArgumentOutOfRangeException>(() => DerivationPaths.ScriptPubKeyTypeFor((ScriptKind)999));
+    }
+
+    [Fact]
+    public void NegativeAccountOrIndex_IsRejected()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => DerivationPaths.AccountPath(ScriptKind.NativeSegwit, ChainProfiles.Mainnet, -1));
+        Assert.Throws<ArgumentOutOfRangeException>(() => DerivationPaths.AddressSubPath(false, -1));
+    }
+
+    [Fact]
+    public void AccountPath_UsesCoinTypeFromProfile()
+    {
+        Assert.Equal("84'/746'/0'", DerivationPaths.AccountPath(ScriptKind.NativeSegwit, ChainProfiles.Mainnet).ToString());
+        Assert.Equal("49'/746'/2'", DerivationPaths.AccountPath(ScriptKind.WrappedSegwit, ChainProfiles.Mainnet, 2).ToString());
+    }
+
+    [Fact]
+    public void KindFor_MapsAddressTypes()
+    {
+        var net = PalladiumNetworks.Regtest;
+        var key = new Key();
+        Assert.Equal(ScriptKind.NativeSegwit, DerivationPaths.KindFor(key.PubKey.GetAddress(ScriptPubKeyType.Segwit, net)));
+        Assert.Equal(ScriptKind.WrappedSegwit, DerivationPaths.KindFor(key.PubKey.GetAddress(ScriptPubKeyType.SegwitP2SH, net)));
+        Assert.Equal(ScriptKind.Legacy, DerivationPaths.KindFor(key.PubKey.GetAddress(ScriptPubKeyType.Legacy, net)));
+        Assert.Equal(ScriptKind.Taproot, DerivationPaths.KindFor(key.PubKey.GetAddress(ScriptPubKeyType.TaprootBIP86, net)));
+        Assert.Equal(ScriptKind.NativeSegwitMultisig,
+            DerivationPaths.KindFor(key.PubKey.WitHash.ScriptPubKey.WitHash.GetAddress(net)));
+    }
+}