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/ZstdBuild.cmake

@@ -0,0 +1,42 @@
+# ################################################################
+# ZSTD Build Targets Configuration
+# ################################################################
+
+# Always build the library first (this defines ZSTD_BUILD_STATIC/SHARED options)
+add_subdirectory(lib)
+
+# Validate build configuration after lib options are defined
+if(ZSTD_BUILD_PROGRAMS)
+    if(NOT ZSTD_BUILD_STATIC AND NOT ZSTD_PROGRAMS_LINK_SHARED)
+        message(SEND_ERROR "Static library required to build zstd CLI programs")
+    elseif(NOT ZSTD_BUILD_SHARED AND ZSTD_PROGRAMS_LINK_SHARED)
+        message(SEND_ERROR "Shared library required to build zstd CLI programs")
+    endif()
+endif()
+
+if(ZSTD_BUILD_TESTS AND NOT ZSTD_BUILD_STATIC)
+    message(SEND_ERROR "Static library required to build test suite")
+endif()
+
+# Add programs if requested
+if(ZSTD_BUILD_PROGRAMS)
+    add_subdirectory(programs)
+endif()
+
+# Add tests if requested
+if(ZSTD_BUILD_TESTS)
+    enable_testing()
+    add_subdirectory(tests)
+endif()
+
+# Add contrib utilities if requested
+if(ZSTD_BUILD_CONTRIB)
+    add_subdirectory(contrib)
+endif()
+
+# Clean-all target for thorough cleanup
+add_custom_target(clean-all
+    COMMAND ${CMAKE_BUILD_TOOL} clean
+    COMMAND ${CMAKE_COMMAND} -E remove_directory ${CMAKE_BINARY_DIR}/
+    COMMENT "Performing complete clean including build directory"
+)