Add native Windows support, CMake improvements, and test fixtures

[Schryzon]

Summary

This PR adds full native Windows compatibility to cb2util so it can be compiled and run seamlessly using CMake (MSVC / MinGW), GCC / Make, and Zig without requiring POSIX system headers or curl during compilation.

Key Changes

• Standalone include/elf.h: Included 32-bit ELF header definitions (Elf32_Ehdr, Elf32_Phdr, Elf32_Shdr) so pcb.c compiles out of the box on non-Linux platforms without fetching via curl.
• Portable getopt implementation: Added lightweight include/getopt.h and getopt.c for compilers lacking POSIX <getopt.h> (e.g. MSVC).
• C99 Compatibility: Replaced GCC zero-length array extensions (uint8_t data[0]) with standard C99 flexible array members (uint8_t data[]) in pcb.c, cbc.c, and cheats.c.
• CMake & Makefile Updates: Added CMakeLists.txt, added automatic FetchContent fallback for zlib when system zlib is absent, and updated Makefile for Windows executable targets (cb2util.exe).
• Test Suite Updates: Added additional CBC test fixtures into test/cbc/.
• Documentation: Updated README.md with Windows installation and build instructions.

Verification

• Tested compilation on Windows with MSVC 19.44 (Visual Studio 2022) via CMake.
• Tested compilation on Windows with MinGW GCC under strict flags (-Wall -Werror).
• Verified execution of cb2util.exe on sample files, passing verification, extraction, and decryption tests.