feat: add ProcessedUidStore with persistence and pruning

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/SpamGuard.Tests/State/ProcessedUidStoreTests.cs

@@ -0,0 +1,70 @@
+// tests/SpamGuard.Tests/State/ProcessedUidStoreTests.cs
+namespace SpamGuard.Tests.State;
+
+using SpamGuard.State;
+
+public class ProcessedUidStoreTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly ProcessedUidStore _store;
+
+    public ProcessedUidStoreTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"spamguard-test-{Guid.NewGuid()}");
+        Directory.CreateDirectory(_tempDir);
+        _store = new ProcessedUidStore(_tempDir);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+            Directory.Delete(_tempDir, true);
+    }
+
+    [Fact]
+    public void Contains_ReturnsFalse_WhenUidNotAdded()
+    {
+        Assert.False(_store.Contains(42));
+    }
+
+    [Fact]
+    public void Contains_ReturnsTrue_AfterAdd()
+    {
+        _store.Add(42);
+        Assert.True(_store.Contains(42));
+    }
+
+    [Fact]
+    public void Add_IsDuplicate_DoesNotThrow()
+    {
+        _store.Add(42);
+        _store.Add(42);
+        Assert.True(_store.Contains(42));
+    }
+
+    [Fact]
+    public void Save_And_Load_RoundTrips()
+    {
+        _store.Add(1);
+        _store.Add(2);
+        _store.Add(3);
+        _store.Save();
+
+        var loaded = new ProcessedUidStore(_tempDir);
+        Assert.True(loaded.Contains(1));
+        Assert.True(loaded.Contains(2));
+        Assert.True(loaded.Contains(3));
+        Assert.False(loaded.Contains(4));
+    }
+
+    [Fact]
+    public void Prune_RemovesOldEntries()
+    {
+        _store.Add(1, DateTime.UtcNow.AddDays(-31));
+        _store.Add(2, DateTime.UtcNow);
+        _store.Prune(TimeSpan.FromDays(30));
+
+        Assert.False(_store.Contains(1));
+        Assert.True(_store.Contains(2));
+    }
+}