feat: add mcpplibs.xpkg@0.0.39 (libxpkg) (#7)

C++23 reference implementation of the xpkg V1 spec. Five modules
under the mcpplibs.xpkg.* namespace (root + loader + index +
executor + lua_stdlib) build into a single libxpkg.a. Depends on
mcpplibs.capi.lua@0.0.3 indexed in this same registry.

Tarball: https://github.com/openxlings/libxpkg/archive/refs/tags/v0.0.39.tar.gz
SHA256:  292d6a85da95b3615cc96f8e2e64dbe7767d059d8a8e9422bbc72db648f81f71

Author: Sunrisepeak

pkgs/m/mcpplibs.xpkg.lua

@@ -0,0 +1,32 @@
+-- Form A descriptor: the upstream repo ships its own mcpp.toml from
+-- v0.0.39 onwards, so we omit the `mcpp` field — mcpp default-look-up
+-- finds <verdir>/libxpkg-<tag>/mcpp.toml inside the GitHub tarball wrap.
+package = {
+    spec        = "1",
+    name        = "mcpplibs.xpkg",
+    description = "C++23 reference implementation of the xpkg V1 spec — `import mcpplibs.xpkg;`",
+    licenses    = {"Apache-2.0"},
+    repo        = "https://github.com/openxlings/libxpkg",
+    type        = "package",
+
+    xpm = {
+        linux = {
+            ["0.0.39"] = {
+                url    = "https://github.com/openxlings/libxpkg/archive/refs/tags/v0.0.39.tar.gz",
+                sha256 = "292d6a85da95b3615cc96f8e2e64dbe7767d059d8a8e9422bbc72db648f81f71",
+            },
+        },
+        macosx = {
+            ["0.0.39"] = {
+                url    = "https://github.com/openxlings/libxpkg/archive/refs/tags/v0.0.39.tar.gz",
+                sha256 = "292d6a85da95b3615cc96f8e2e64dbe7767d059d8a8e9422bbc72db648f81f71",
+            },
+        },
+        windows = {
+            ["0.0.39"] = {
+                url    = "https://github.com/openxlings/libxpkg/archive/refs/tags/v0.0.39.tar.gz",
+                sha256 = "292d6a85da95b3615cc96f8e2e64dbe7767d059d8a8e9422bbc72db648f81f71",
+            },
+        },
+    },
+}