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/Storage/StorageEdgeCaseTests.cs

@@ -0,0 +1,87 @@
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Crypto;
+using PalladiumWallet.Core.Storage;
+
+namespace PalladiumWallet.Tests.Storage;
+
+[Collection("AppPaths")] // DefaultDataRoot reads the static override seam
+public class StorageEdgeCaseTests
+{
+    [Fact]
+    public void SyncCache_RoundTripsEveryField_ThroughEncryptedStore()
+    {
+        var doc = new WalletDocument
+        {
+            Network = "regtest",
+            ScriptKind = nameof(ScriptKind.NativeSegwit),
+            Cache = new SyncCache
+            {
+                TipHeight = 123, ConfirmedSats = 5_000, UnconfirmedSats = 7, ImmatureSats = 100,
+                PendingVerificationSats = 200, SpendableSats = 4_700, NextReceiveIndex = 3, NextChangeIndex = 2,
+                History = [new CachedTx { Txid = "aa", Height = 10, DeltaSats = -50, Verified = true }],
+                Utxos = [new CachedUtxo { Txid = "bb", Vout = 1, ValueSats = 99, Address = "a", IsChange = true,
+                    AddressIndex = 4, Height = 11, IsCoinbase = true, Frozen = true, Verified = true }],
+                Addresses = [new CachedAddress { Address = "a", IsChange = true, Index = 4, BalanceSats = 99, TxCount = 2 }],
+                RawTxHex = new() { ["aa"] = "0102" },
+                VerifiedAt = new() { ["aa"] = 10 },
+                BlockHeaders = new() { [10] = "00" },
+                AnchoredUpTo = new() { [100] = 150 },
+            },
+        };
+        var path = Path.Combine(Path.GetTempPath(), $"plm-cache-{Guid.NewGuid()}.json");
+        try
+        {
+            WalletStore.Save(doc, path, "pw");
+            var c = WalletStore.Load(path, "pw").Cache!;
+
+            Assert.Equal((123, 5_000, 7, 100, 200, 4_700, 3, 2),
+                (c.TipHeight, c.ConfirmedSats, c.UnconfirmedSats, c.ImmatureSats, c.PendingVerificationSats,
+                 c.SpendableSats, c.NextReceiveIndex, c.NextChangeIndex));
+            Assert.True(c.History[0].Verified);
+            Assert.True(c.Utxos[0].Frozen && c.Utxos[0].IsCoinbase && c.Utxos[0].IsChange);
+            Assert.Equal(2, c.Addresses[0].TxCount);
+            Assert.Equal("0102", c.RawTxHex!["aa"]);
+            Assert.Equal(10, c.VerifiedAt!["aa"]);
+            Assert.Equal("00", c.BlockHeaders![10]);
+            Assert.Equal(150, c.AnchoredUpTo![100]);
+        }
+        finally { File.Delete(path); }
+    }
+
+    [Theory]
+    [InlineData("{ not json")]
+    [InlineData("")]
+    [InlineData("null")]
+    public void AppConfig_FallsBackToDefaults_OnCorruptedOrEmptyFile(string content)
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"plm-config-{Guid.NewGuid()}.json");
+        try
+        {
+            File.WriteAllText(path, content);
+            var config = AppConfig.Load(path);
+            Assert.NotNull(config);
+            Assert.Null(config.LastServerHost);
+        }
+        finally { File.Delete(path); }
+    }
+
+    [Fact]
+    public void AppConfig_Load_ReturnsDefaultsWhenFileIsMissing() =>
+        Assert.NotNull(AppConfig.Load(Path.Combine(Path.GetTempPath(), $"missing-{Guid.NewGuid()}.json")));
+
+    [Fact]
+    public void DefaultDataRoot_FollowsPlatformConvention()
+    {
+        var saved = AppPaths.DefaultRootOverride;
+        AppPaths.DefaultRootOverride = null;
+        try
+        {
+            var root = AppPaths.DefaultDataRoot();
+            if (OperatingSystem.IsWindows())
+                Assert.EndsWith(AppPaths.AppDirName, root);
+            else
+                Assert.EndsWith(".palladium-wallet", root);
+        }
+        finally { AppPaths.DefaultRootOverride = saved; }
+    }
+}