feat(pkgs): llvm-musl-libcxx — musl-targeted libc++ static runtime payload

[yspbwx2010]

Summary

• New data-only package llvm-musl-libcxx: the musl-targeted libc++/libc++abi/libunwind static archives, headers and std.cppm/std.compat.cppm module sources, one asset per target arch (x86_64 + aarch64, each sha256-pinned).
• Why: a clang frontend carries no target libc, so the LLVM family cannot serve *-linux-musl without a libc++ configured for the musl target (a host libc++'s __config_site describes a different ABI) and std module sources built over it. crt/libgcc/libc are NOT duplicated here — the engine side points clang at the existing musl-gcc payload via --gcc-toolchain.
• Built from llvm-project release/22.x runtimes, the clang 22.1.8 payload cross-driving into each target's musl sysroot. Static-only: the musl target is a fully static ELF world.
• Umbrella xvm node like libcxx-headers (a second copy of the standard library must not reach the host sysroot).
• Assets staged on the contributor fork's release until migrated to xlings-res (sha256 pins the bytes); no ci block for the same reason libcxx-headers has none — the payload is rebuilt per llvm release, not re-downloaded.

Engine-side consumer: mcpp-community/mcpp#492 (adds the llvmSysroot target-table column that resolves this payload); tracking issue mcpp-community/mcpp#491. Design doc: .agents/docs/2026-08-23-llvm-musl-libcxx-package-design.md.

Test plan

• pytest tests/ -m 'static or isolation': 1887 passed, 9 skipped (12 new tests in tests/l/test_llvm_musl_libcxx.py, including the both-arches-sha256 and static-only assertions)
• Isolated-XLINGS_HOME full lifecycle: config --add-xpkg → install (download + sha256 + install-hook asserts on algorithm header / libc++.a / std.cppm) → remove, all green
• End-to-end via the engine branch (mcpp#492) equivalent: fully static x86_64 binary (native run) and aarch64 binary (qemu) produced from this payload

[yspbwx2010]

Assets now live in a dedicated payload repo — cloud-teahouse/llvm-musl-libcxx (source + per-release archives with sha256 in the notes) — replacing the earlier staging spot; same bytes, same sha256. Descriptor updated and the full install/remove lifecycle re-verified against the new URL (12/12 package tests green).

[yspbwx2010]

Superseded: mcpp now ships native --target <arch>-linux-musl support via openkal-llvm-runtime (verified end-to-end on mcpp v2026.8.26.2, see mcpp-community/mcpp#492). The prebuilt llvm-musl-libcxx payload this descriptor packaged is no longer needed — closing. Thanks for the review bandwidth.