feat(xpkg): publish libxpkg 0.0.55

[Sunrisepeak]

Publishes the source-library release from openxlings/libxpkg#39 for downstream xlings issues openxlings/xlings#513 and openxlings/xlings#514.

Before this descriptor change, the annotated 0.0.55 source tag existed but a minimal public mcpp build still failed with E_NOT_FOUND: the mcpplibs index advertised mcpplibs.xpkg only through 0.0.54. A source tag is therefore necessary but not sufficient for package-manager resolution.

The 0.0.55 entry is added to all three platform blocks (linux, macosx, windows) with identical source metadata:

• GLOBAL: https://github.com/openxlings/libxpkg/archive/refs/tags/0.0.55.tar.gz
• CN: https://gitcode.com/mcpp-res/xpkg/releases/download/0.0.55/xpkg-0.0.55.tar.gz
• SHA-256: d388c1dd3a531a8e03d304ee2f8dac7a693904daa45de4c400eea7169418fb45

Release evidence:

• libxpkg annotated tag object dc0b08ad01a401ecc7ee2a4ca4290d342c17a320 peels to merged commit 7ecf628fe8cc19fa3419abe59e9d4fba1d93d2d8.
• Two independent GLOBAL GETs returned 140,574 bytes and the same hash.
• GitCode release/asset preflight returned 404/404; local gtc created the release and uploaded the exact archive once.
• CN GET returned HTTP 200 and 140,574 bytes; cmp and SHA-256 prove it is byte-identical to GLOBAL.
• The archive root is libxpkg-0.0.55/, and its mcpp.toml reports version 0.0.55.

Focused validation passed:

• mcpp xpkg parse pkgs/x/xpkg.lua --json
• tests/check_platform_version_parity.lua
• tests/check_mirror_urls.lua
• exact new CN URL/hash count is one in tests/list_cn_urls.lua
• git diff --check

After merge, the downstream gate will refresh the public mcpplibs index and require a minimal mcpp add mcpplibs.xpkg@0.0.55 plus build/lock/source-version probe before xlings consumes the release.