fix: Update README to reflect current Zig implementation

- Changed build system from Rust/Cargo to Zig
- Updated installation instructions to use 'zig build'
- Removed external LLVM dependency requirement
- Updated command examples to use cursed-compiler binary
- Fixed project structure to show src-zig/ directory
- Updated development status from 'Stage 1 Rust' to mature Zig implementation
- Emphasized cross-platform support including Windows
- Reflects current reality of full compiler with LLVM backend