feat(loader): support target platform evaluation

[Sunrisepeak]

Summary

• add LoaderContext { platform, arch } and a platform-aware load_package overload
• evaluate legacy is_host, is_plat, is_arch, os.host, os.arch, and _RUNTIME conditionals inside libxpkg
• preserve the existing context-free load_package(path) API
• bump libxpkg to 0.0.44

This is required before openxlings/xlings#356 can delete its duplicate platform Lua parser and make libxpkg the only parse/compat/normalization entry point.

Validation

• mcpp build passes
• platform-context loader regression passes for Linux x86_64 and macOS aarch64
• loader/compat suite: 27/27 pass
• the full suite retains the same four pre-existing elfpatch executor failures reproduced on clean main