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/Core/Asset/CMakeLists.txt

@@ -0,0 +1,35 @@
+# ============================================================
+# Core/Asset Tests
+# ============================================================
+
+set(ASSET_TEST_SOURCES
+    test_iresource.cpp
+    test_resource_types.cpp
+    test_resource_guid.cpp
+    test_resource_handle.cpp
+    test_resource_cache.cpp
+    test_resource_dependency.cpp
+)
+
+add_executable(asset_tests ${ASSET_TEST_SOURCES})
+
+if(MSVC)
+    set_target_properties(asset_tests PROPERTIES
+        LINK_FLAGS "/NODEFAULTLIB:libcpmt.lib /NODEFAULTLIB:libcmt.lib"
+    )
+endif()
+
+target_link_libraries(asset_tests
+    PRIVATE
+    XCEngine
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(asset_tests PRIVATE
+    ${CMAKE_SOURCE_DIR}/engine/include
+    ${CMAKE_SOURCE_DIR}/tests/fixtures
+)
+
+include(GoogleTest)
+gtest_discover_tests(asset_tests)