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/ZstdOptions.cmake

@@ -0,0 +1,68 @@
+# ################################################################
+# ZSTD Build Options Configuration
+# ################################################################
+
+# Legacy support configuration
+option(ZSTD_LEGACY_SUPPORT "Enable legacy format support" ON)
+
+if(ZSTD_LEGACY_SUPPORT)
+    message(STATUS "ZSTD_LEGACY_SUPPORT enabled")
+    set(ZSTD_LEGACY_LEVEL 5 CACHE STRING "Legacy support level")
+    add_definitions(-DZSTD_LEGACY_SUPPORT=${ZSTD_LEGACY_LEVEL})
+else()
+    message(STATUS "ZSTD_LEGACY_SUPPORT disabled")
+    add_definitions(-DZSTD_LEGACY_SUPPORT=0)
+endif()
+
+# Platform-specific options
+if(APPLE)
+    option(ZSTD_FRAMEWORK "Build as Apple Framework" OFF)
+endif()
+
+# Android-specific configuration
+if(ANDROID)
+    set(ZSTD_MULTITHREAD_SUPPORT_DEFAULT OFF)
+    # Handle old Android API levels
+    if((NOT ANDROID_PLATFORM_LEVEL) OR (ANDROID_PLATFORM_LEVEL VERSION_LESS 24))
+        message(STATUS "Configuring for old Android API - disabling fseeko/ftello")
+        add_compile_definitions(LIBC_NO_FSEEKO)
+    endif()
+else()
+    set(ZSTD_MULTITHREAD_SUPPORT_DEFAULT ON)
+endif()
+
+# Multi-threading support
+option(ZSTD_MULTITHREAD_SUPPORT "Enable multi-threading support" ${ZSTD_MULTITHREAD_SUPPORT_DEFAULT})
+
+if(ZSTD_MULTITHREAD_SUPPORT)
+    message(STATUS "Multi-threading support enabled")
+else()
+    message(STATUS "Multi-threading support disabled")
+endif()
+
+# Build component options
+option(ZSTD_BUILD_PROGRAMS "Build command-line programs" ON)
+option(ZSTD_BUILD_CONTRIB "Build contrib utilities" OFF)
+option(ZSTD_PROGRAMS_LINK_SHARED "Link programs against shared library" OFF)
+
+# Test configuration
+if(BUILD_TESTING)
+    set(ZSTD_BUILD_TESTS_default ON)
+else()
+    set(ZSTD_BUILD_TESTS_default OFF)
+endif()
+option(ZSTD_BUILD_TESTS "Build test suite" ${ZSTD_BUILD_TESTS_default})
+
+# MSVC-specific options
+if(MSVC)
+    option(ZSTD_USE_STATIC_RUNTIME "Link to static runtime libraries" OFF)
+endif()
+
+# C++ support (needed for tests)
+set(ZSTD_ENABLE_CXX ${ZSTD_BUILD_TESTS})
+if(ZSTD_ENABLE_CXX)
+    enable_language(CXX)
+endif()
+
+# Set global definitions
+add_definitions(-DXXH_NAMESPACE=ZSTD_)