feat: system config DSL, services, CLI with rustc diagnostics

- System config parser (system/packages/services/boot/users blocks)
- Boot block: kernel, init, efi, swap, bootloader, root partition
- System-wide features, rollback config, swap partition
- Service block: per-init variants (runit, s6, etc.), system init selection via boot.init
- Feature-gated dependencies, config values accept numbers/bools/strings
- CLI: parse-package, parse-config, validate subcommands
- Rustc-style error diagnostics with source context, carets, help text
- Diagnostic module with ANSI color output

CMakeLists.txt

@@ -27,7 +27,9 @@ add_executable(kappa
   src/paths.cpp
   src/dsl/lexer.cpp
   src/dsl/parser.cpp
+  src/dsl/system.cpp
   src/eval/vars.cpp
+  src/cli/diagnostic.cpp
 )
 target_include_directories(kappa PRIVATE include)
 target_link_libraries(kappa PRIVATE tomlplusplus::tomlplusplus)