docs: update TEST_SPEC.md and README.md to reflect new directory structure

- TEST_SPEC.md: Updated test directory structure to reflect Core/Asset,
  Core/IO, and Resources/<Type> subdirectories
- TEST_SPEC.md: Updated module names and test counts (852 total)
- TEST_SPEC.md: Updated build commands for new Resources subdirectories
- README.md: Updated engine structure with Core/Asset/ and Core/IO/
- README.md: Updated Resources section with layered architecture
- README.md: Updated test coverage table with accurate counts

tests/fixtures/MathFixtures.h

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <gtest/gtest.h>
-#include <XCEngine/Math/Math.h>
+#include <XCEngine/Core/Math/Math.h>
 
 namespace XCEngine {
 namespace Math {