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/Wallet/UtxoSpendabilityTests.cs

@@ -0,0 +1,62 @@
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Storage;
+using PalladiumWallet.Core.Wallet;
+
+namespace PalladiumWallet.Tests.Wallet;
+
+/// <summary>
+/// Boundary tests of the single spendability gate: heights 0 and -1 are mempool, never confirmed.
+/// </summary>
+public class UtxoSpendabilityTests
+{
+    private static readonly ChainProfile Profile = ChainProfiles.Mainnet;
+
+    private static CachedUtxo Utxo(int height, bool verified = true, bool frozen = false, bool coinbase = false) => new()
+    {
+        Txid = "aa", Vout = 0, ValueSats = 1, Address = "a", Height = height,
+        Verified = verified, Frozen = frozen, IsCoinbase = coinbase,
+    };
+
+    [Theory]
+    [InlineData(0)]    // unconfirmed
+    [InlineData(-1)]   // unconfirmed with unconfirmed parents (Electrum convention)
+    public void MempoolUtxos_HaveZeroConfirmations_AndAreNeverSpendable(int height)
+    {
+        var utxo = Utxo(height, verified: true);
+        Assert.Equal(0, utxo.Confirmations(tipHeight: 1_000));
+        Assert.False(utxo.IsSpendable(Profile, tipHeight: 1_000));
+    }
+
+    [Theory]
+    [InlineData(100, 100, 1)]
+    [InlineData(100, 105, 6)]
+    public void Confirmations_CountTheBlockItself(int height, int tip, int expected) =>
+        Assert.Equal(expected, Utxo(height).Confirmations(tip));
+
+    [Fact]
+    public void RegularOutput_BecomesSpendableExactlyAtMinConfirmations()
+    {
+        var min = Profile.MinConfirmations;
+        var utxo = Utxo(height: 100);
+        Assert.False(utxo.IsSpendable(Profile, tipHeight: 100 + min - 2)); // min-1 confirmations
+        Assert.True(utxo.IsSpendable(Profile, tipHeight: 100 + min - 1));  // min confirmations
+    }
+
+    [Fact]
+    public void Coinbase_NeedsMaturityPlusOne_Confirmations()
+    {
+        var utxo = Utxo(height: 100, coinbase: true);
+        var needed = Profile.CoinbaseMaturity + 1;
+        Assert.Equal(needed, utxo.RequiredConfirmations(Profile));
+        Assert.False(utxo.IsSpendable(Profile, tipHeight: 100 + needed - 2));
+        Assert.True(utxo.IsSpendable(Profile, tipHeight: 100 + needed - 1));
+    }
+
+    [Fact]
+    public void UnverifiedOrFrozen_AreNeverSpendable_EvenWhenDeeplyConfirmed()
+    {
+        Assert.False(Utxo(100, verified: false).IsSpendable(Profile, tipHeight: 10_000));
+        Assert.False(Utxo(100, frozen: true).IsSpendable(Profile, tipHeight: 10_000));
+        Assert.True(Utxo(100).IsSpendable(Profile, tipHeight: 10_000));
+    }
+}