feat(xpkg): add libxpkg 0.0.44

[Sunrisepeak]

Summary

• register mcpplibs.xpkg 0.0.44 for Linux, macOS, and Windows
• publish and reference the byte-identical mcpp-res/xpkg CN mirror asset
• add mcpplibs.xpkg to the README Form-A examples and record resource verification
• intentionally skip 0.0.43 in favor of the final 0.0.44 release

Package shape

External Form-A C++23 module repository. Upstream ships mcpp.toml; no inline build metadata or source-gated feature is added.

Resource evidence

• upstream release: https://github.com/openxlings/libxpkg/releases/tag/v0.0.44
• release commit: 343a1fd9a697ba868af209b43862899b05a580c8
• GLOBAL downloaded independently twice
• CN public download: HTTP 200
• all three files: 127252 bytes
• SHA256: 152a27425ba418312182bddb316c8c5bc636bbd8a37faf30e75390dc844e2e95
• GLOBAL/CN: byte-identical by cmp

Verification

• full Lua descriptor and mirror lint: pass
• CI-pinned mcpp 0.0.87 strict parse: 44/44 descriptors pass
• new CN URL: HTTP 200, expected SHA256, byte-identical

No new workspace example is included because mcpplibs is the default remote index name: an unpublished local path override resolves metadata locally but the xlings fetch subprocess still selects the published same-name remote registry. A git-source example would bypass this descriptor and provide false coverage. The design record documents the observed boundary; the existing cross-platform workspace matrix remains the regression gate.