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/ZstdVersion.cmake

@@ -0,0 +1,31 @@
+# ################################################################
+# ZSTD Version Configuration
+# ################################################################
+
+# Setup CMake policy version
+set(ZSTD_MAX_VALIDATED_CMAKE_MAJOR_VERSION "3")
+set(ZSTD_MAX_VALIDATED_CMAKE_MINOR_VERSION "13")
+
+# Determine appropriate policy version
+if("${ZSTD_MAX_VALIDATED_CMAKE_MAJOR_VERSION}" EQUAL "${CMAKE_MAJOR_VERSION}" AND
+   "${ZSTD_MAX_VALIDATED_CMAKE_MINOR_VERSION}" GREATER "${CMAKE_MINOR_VERSION}")
+    set(ZSTD_CMAKE_POLICY_VERSION "${CMAKE_VERSION}")
+else()
+    set(ZSTD_CMAKE_POLICY_VERSION "${ZSTD_MAX_VALIDATED_CMAKE_MAJOR_VERSION}.${ZSTD_MAX_VALIDATED_CMAKE_MINOR_VERSION}.0")
+endif()
+
+cmake_policy(VERSION ${ZSTD_CMAKE_POLICY_VERSION})
+
+# Parse version from header file
+include(GetZstdLibraryVersion)
+GetZstdLibraryVersion(${LIBRARY_DIR}/zstd.h zstd_VERSION_MAJOR zstd_VERSION_MINOR zstd_VERSION_PATCH)
+
+# Set version variables
+set(ZSTD_SHORT_VERSION "${zstd_VERSION_MAJOR}.${zstd_VERSION_MINOR}")
+set(ZSTD_FULL_VERSION "${zstd_VERSION_MAJOR}.${zstd_VERSION_MINOR}.${zstd_VERSION_PATCH}")
+
+# Project metadata
+set(zstd_HOMEPAGE_URL "https://facebook.github.io/zstd")
+set(zstd_DESCRIPTION "Zstandard is a real-time compression algorithm, providing high compression ratios.")
+
+message(STATUS "ZSTD VERSION: ${zstd_VERSION_MAJOR}.${zstd_VERSION_MINOR}.${zstd_VERSION_PATCH}")