perf: optimize ExcelIO tests with fixture-based consolidation

- Add WorkbookFixtureBase and 4 concrete fixtures for shared workbooks
- Add ExcelTestHelpers with shared utility methods
- Create Integration/ folder with 7 fixture-based test classes:
  - MinimalSearchTests (5 tests)
  - SearchResultsSheetTests (5 tests)
  - MisInfoSheetTests (11 tests)
  - InvestigationSheetTests (7 tests)
  - ProtectionAndStyleTests (7 tests)
  - LegacyFormatTests (5 tests)
  - LargeDataSetTests (1 test)
- Delete redundant ExcelExportIntegrationTests.cs (26 tests)
- Delete redundant LegacyComparisonTests.cs (16 tests)
- Reduce workbook generations from ~42 to 4 fixtures
- Test runtime reduced from ~18 min to ~4 min (76% improvement)
- All 122 ExcelIO tests pass

NEW/tests/JdeScoping.ExcelIO.Tests/Integration/LegacyFormatTests.cs

@@ -0,0 +1,41 @@
+using Shouldly;
+using Xunit;
+using ExcelFormats = JdeScoping.ExcelIO.Formatting.ExcelFormats;
+
+namespace JdeScoping.ExcelIO.Tests.Integration;
+
+/// <summary>
+/// Tests for ExcelFormats constants - no workbook generation needed.
+/// </summary>
+public class LegacyFormatTests
+{
+    [Fact]
+    public void TimestampFormat_MatchesLegacy()
+    {
+        ExcelFormats.TimestampFormat.ShouldBe("[$-409]m/d/yy h:mm AM/PM;@");
+    }
+
+    [Fact]
+    public void DateFormat_MatchesLegacyPattern()
+    {
+        ExcelFormats.DateFormat.ShouldContain("MM/dd/yyyy");
+    }
+
+    [Fact]
+    public void WrappedColumnWidth_MatchesLegacy()
+    {
+        ExcelFormats.WrappedColumnWidth.ShouldBe(65);
+    }
+
+    [Fact]
+    public void CriteriaPaddingFactor_MatchesLegacy()
+    {
+        ExcelFormats.CriteriaPaddingFactor.ShouldBe(1.15);
+    }
+
+    [Fact]
+    public void DataPaddingFactor_MatchesLegacy()
+    {
+        ExcelFormats.DataPaddingFactor.ShouldBe(1.30);
+    }
+}