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/LargeDataSetTests.cs

@@ -0,0 +1,24 @@
+using ClosedXML.Excel;
+using JdeScoping.ExcelIO.Tests.Fixtures;
+using Shouldly;
+using Xunit;
+
+namespace JdeScoping.ExcelIO.Tests.Integration;
+
+public class LargeDataSetTests : IClassFixture<LargeDataSetFixture>
+{
+    private readonly XLWorkbook _workbook;
+
+    public LargeDataSetTests(LargeDataSetFixture fixture)
+    {
+        _workbook = fixture.Workbook;
+    }
+
+    [Fact]
+    public void TableRowCount_Is1001()
+    {
+        var sheet = _workbook.Worksheet("Search Results");
+        var table = sheet.Tables.First();
+        table.RowCount().ShouldBe(1001); // 1 header + 1000 data rows
+    }
+}