docs: define xpkg v1 runtime boundary

[Sunrisepeak]

Summary

• document the xpkg V1 package-script boundary as libxpkg + standard Lua + xpkg spec + xpkg built-in APIs
• keep package authoring guidance away from xmake API terminology
• remove the CI workaround that manually staged mcpp dependency BMI/archive files and patched build.ninja
• require native mcpp build / mcpp test plus a real xmake build xpkg aggregate build with mcpp 0.0.30

Root cause notes

• Native mcpp build originally failed because mcpp 0.0.29/0.0.30 looked for the qualified descriptor mcpplibs.capi.lua.lua, missed the existing capi.lua.lua, then fell back to compat.lua.lua.
• Fixed upstream in fix: add qualified capi lua descriptor alias mcpplibs/mcpp-index#16 by adding the qualified descriptor alias; no libxpkg CI workaround is kept.
• The xmake CI failure was separate: calling $GCC_ROOT/bin/gcc directly bypassed the xlings GCC shim/sysroot setup, so xmake package builds could not find glibc CRT/header files. CI now uses the xlings gcc/g++ shims for compile/link and system /usr/bin/ar//usr/bin/as for archive/assemble.

Test plan

• mcpp build
• mcpp test -- --gtest_filter=-ExecutorTest.ApplyElfpatchAuto_*
• xmake f -c -m release --toolchain=gcc --cc="$HOME/.xlings/subos/current/bin/gcc" --cxx="$HOME/.xlings/subos/current/bin/g++" --ld="$HOME/.xlings/subos/current/bin/g++" --sh="$HOME/.xlings/subos/current/bin/g++" --ar=/usr/bin/ar --as=/usr/bin/as --policies=build.c++.modules.std -y
• xmake build -y -j$(nproc) xpkg
• CI build + test (linux x86_64) passed on run 26472375115