refactor: modularize CMakeLists.txt for better maintainability

- Split monolithic 235-line CMakeLists.txt into focused modules
- Main file reduced to 78 lines with clear section organization
- Created 5 specialized modules:
  * ZstdVersion.cmake - CMake policies and version management
  * ZstdOptions.cmake - Build options and platform configuration
  * ZstdDependencies.cmake - External dependency management
  * ZstdBuild.cmake - Build targets and validation
  * ZstdPackage.cmake - Package configuration generation

Benefits:
- Improved readability and maintainability
- Better separation of concerns
- Easier debugging and modification
- Preserved 100% backward compatibility
- All existing build options and targets unchanged

The refactored build system passes all tests and maintains
identical functionality while being much easier to understand
and maintain.

build/cmake/CMakeModules/ZstdPackage.cmake

@@ -0,0 +1,42 @@
+# ################################################################
+# ZSTD Package Configuration
+# ################################################################
+
+include(CMakePackageConfigHelpers)
+
+# Generate version file
+write_basic_package_version_file(
+    "${CMAKE_CURRENT_BINARY_DIR}/zstdConfigVersion.cmake"
+    VERSION ${zstd_VERSION}
+    COMPATIBILITY SameMajorVersion
+)
+
+# Export targets for build directory
+export(EXPORT zstdExports
+    FILE "${CMAKE_CURRENT_BINARY_DIR}/zstdTargets.cmake"
+    NAMESPACE zstd::
+)
+
+# Configure package for installation
+set(ConfigPackageLocation ${CMAKE_INSTALL_LIBDIR}/cmake/zstd)
+
+# Install exported targets
+install(EXPORT zstdExports
+    FILE zstdTargets.cmake
+    NAMESPACE zstd::
+    DESTINATION ${ConfigPackageLocation}
+)
+
+# Configure and install package config file
+configure_package_config_file(
+    zstdConfig.cmake.in
+    "${CMAKE_CURRENT_BINARY_DIR}/zstdConfig.cmake"
+    INSTALL_DESTINATION ${ConfigPackageLocation}
+)
+
+# Install config files
+install(FILES
+    "${CMAKE_CURRENT_BINARY_DIR}/zstdConfig.cmake"
+    "${CMAKE_CURRENT_BINARY_DIR}/zstdConfigVersion.cmake"
+    DESTINATION ${ConfigPackageLocation}
+)