feat: complete first MITC4 FEAST eigen implementation validation

tests/unit/io/hdf5/eigen_hdf5_results_writer_test.cpp

@@ -1,5 +1,7 @@
+#define NOMINMAX
 #include "fesa/io/hdf5/eigen_hdf5_results_writer.h"
 
+#include <Windows.h>
 #include <gtest/gtest.h>
 #include <hdf5.h>
 
@@ -113,6 +115,22 @@ TEST(EigenHdf5ResultsWriter, WritesModalSchemaAndRejectsTamperedState) {
   const std::string retained{std::istreambuf_iterator<char>{retained_file}, {}};
   EXPECT_TRUE(retained == original);
   retained_file.close();
+  // Denying delete sharing forces the actual atomic replacement to fail after
+  // writing and validating a fresh candidate, while retaining a valid final.
+  const HANDLE lock = CreateFileW(path.c_str(), GENERIC_READ,
+                                  FILE_SHARE_READ | FILE_SHARE_WRITE, nullptr,
+                                  OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
+  ASSERT_NE(lock, INVALID_HANDLE_VALUE);
+  const auto locked_write = writer.Write(path, domain, state, {});
+  EXPECT_NE(CloseHandle(lock), 0);
+  ASSERT_FALSE(locked_write.IsOk());
+  ASSERT_FALSE(locked_write.Diagnostics().empty());
+  EXPECT_EQ(locked_write.Diagnostics()[0].code, "hdf5-finalization-failure");
+  std::ifstream locked_final{path, std::ios::binary};
+  const std::string after_failure{std::istreambuf_iterator<char>{locked_final},
+                                {}};
+  EXPECT_EQ(after_failure, original);
+  locked_final.close();
   ASSERT_TRUE(
       fesa::hdf5_internal::SelfCheckEigenFile(path, domain, state, {}).IsOk());
   const hid_t corrupt_file =