emit xpkg: a universal self-contained package's entry names every system with every image architecture, not the loaders it carries #860

What happens

mcpp emit xpkg --package for a universal package (mcpp pack --mode self-contained --systems linux,macos,windows) writes one block per system, each with every image architecture: archs = {"aarch64", "x86_64"} and a single sha256 for linux, macosx and windows (src/pack/distribution.cppm, the universal branch of the entry writer).

The package carries a loader only for the systems in its mcpp-package.json systems (libexec/okrun/<os>-<arch>/). For mcpp-ide 0.1.0 those are linux-x86_64, linux-aarch64, macos-aarch64 and windows-x86_64. On any other pair, the launcher (bin/mcpp-ide, bin/mcpp-ide.cmd) falls back to okrun on PATH. pkgs/o/okrun.lua is built for the same four pairs. So on an Intel Mac or a Windows arm64 machine, xlings installs the entry and the program does not start.

Expected

For a self-contained universal package, each system's block names only the architectures whose loader the package carries (systems, with macos written as macosx), as per-architecture entries. A package that depends on the runner (another mode, deps = { "xim:okrun" }) has no such information and keeps every image architecture.

Where it was found

The ecosystem review of the mcpp-ide 0.1.1 index entry (openxlings/xim-pkgindex, branch mcppls-0.0.19-mcpp-ide-0.1.1). That entry now restricts macOS to aarch64 and Windows to x86_64 by hand. The fix in the emitter lets a regenerated entry say the same.

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