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/TransactionFactoryLimitsTests.cs

@@ -0,0 +1,74 @@
+using NBitcoin;
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Crypto;
+using PalladiumWallet.Core.Storage;
+using PalladiumWallet.Core.Wallet;
+
+namespace PalladiumWallet.Tests.Wallet;
+
+/// <summary>Spend-policy limits: send-all affordability and the standard-size cap.</summary>
+public class TransactionFactoryLimitsTests
+{
+    private static readonly Network Net = PalladiumNetworks.Regtest;
+
+    private static HdAccount Account()
+    {
+        Assert.True(Bip39.TryParse(
+            "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+            out var mnemonic));
+        return HdAccount.FromMnemonic(mnemonic!, null, ScriptKind.NativeSegwit, ChainProfiles.Regtest);
+    }
+
+    /// <summary>One funding tx with <paramref name="count"/> outputs to receive/0, each a verified UTXO.</summary>
+    private static (List<CachedUtxo>, Dictionary<string, Transaction>) Fund(HdAccount account, int count, long satsEach)
+    {
+        var funding = Net.CreateTransaction();
+        funding.Inputs.Add(new TxIn(new OutPoint(uint256.One, 0)));
+        for (var i = 0; i < count; i++)
+            funding.Outputs.Add(Money.Satoshis(satsEach), account.GetReceiveAddress(0));
+        var txid = funding.GetHash().ToString();
+        var utxos = Enumerable.Range(0, count).Select(i => new CachedUtxo
+        {
+            Txid = txid, Vout = i, ValueSats = satsEach,
+            Address = account.GetReceiveAddress(0).ToString(),
+            AddressIndex = 0, Height = 100, Verified = true,
+        }).ToList();
+        return (utxos, new Dictionary<string, Transaction> { [txid] = funding });
+    }
+
+    [Fact]
+    public void SendAll_WithBalanceBelowTheFee_IsRejectedAsInsufficientFunds()
+    {
+        var account = Account();
+        var (utxos, txs) = Fund(account, 1, 50); // 50 sat cannot pay for a ~110 vB transaction
+
+        var ex = Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(5), amountSats: 0,
+            feeRateSatPerVByte: 2, changeIndex: 0, tipHeight: 1_000, sendAll: true));
+        Assert.Contains("Insufficient funds", ex.Message);
+    }
+
+    [Fact]
+    public void SendAll_OverTheStandardSizeCap_IsRejectedWithGuidance()
+    {
+        var account = Account();
+        // ~68 vB per P2WPKH input: 1,700 inputs exceed the 100,000 vB standard limit.
+        var (utxos, txs) = Fund(account, 1_700, 10_000);
+
+        var ex = Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(5), amountSats: 0,
+            feeRateSatPerVByte: 1, changeIndex: 0, tipHeight: 1_000, sendAll: true));
+        Assert.Contains("Send-all cannot fit in one standard transaction", ex.Message);
+    }
+
+    [Fact]
+    public void AmountAboveTheBalance_IsRejectedAsInsufficientFunds()
+    {
+        var account = Account();
+        var (utxos, txs) = Fund(account, 1, 100_000);
+
+        Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(5), amountSats: 500_000,
+            feeRateSatPerVByte: 2, changeIndex: 0, tipHeight: 1_000));
+    }
+}