[FEATURE] Publish PyPy musllinux wheels (Alpine / musl libc)

Follow-up to #1877, which added CPython 3.11–3.14 musllinux_1_2 wheels. PyPy-on-musl
was deferred there because there is no official pypa musllinux PyPy image.

Why it was deferred

• The quay.io/pypa/musllinux_1_2_{x86_64,aarch64} images ship only CPython under
  /opt/python (3.9–3.15 + free-threaded variants) — no PyPy (verified locally on
  asgard1 while implementing #1877).
• The existing PyPy manylinux aarch64 wheels are built from custom images
  (docker/Dockerfile.pypy-bookworm-manylinux-arm64, …trixie…, FROM pypy:3.11-*,
  Debian/glibc). A musllinux PyPy build needs equivalent Alpine-based custom images.

Scope

Publish pp311 musllinux_1_2 wheels (x86_64 + aarch64) for PyPy 3.11, matching the
CPython musllinux coverage from #1877.

Tasks

• Add custom musllinux PyPy Dockerfile(s) under docker/ (Alpine + PyPy 3.11 + build-base
  • patchelf + auditwheel), mirroring the manylinux PyPy Dockerfiles.
• Add matrix entries (build_type: custom) to wheels-docker.yml (x86_64) and
  wheels-arm64.yml (aarch64) using those images.
• Verify the NVX CFFI extension builds on PyPy-musl (and re-check the PyPy-specific
  PYUBJSON_NO_EXTENSION / build-tools handling used by the manylinux PyPy path).
• Wire release.yml artifacts + add pp311-linux-<arch>-musllinux_1_2 fileset targets.
• Update docs/wheels-inventory.rst.

Related

• #1877 (CPython musllinux wheels).
• zlmdb: the PyPy-musllinux mirror follow-up.

Note: This work was drafted with AI assistance (Claude Code). Initiated and reviewed by me.

Checklist

• I have searched existing issues to avoid duplicates
• I have described the problem clearly
• I have provided use cases
• I have considered alternatives
• I have assessed impact and breaking changes

Start by comparing the existing PyPy manylinux Dockerfiles under docker/ with the CPython musllinux entries in wheels-docker.yml and wheels-arm64.yml. Then trace release.yml and docs/wheels-inventory.rst to identify the artifact and fileset updates. Done means pp311 musllinux_1_2 wheels exist for x86_64 and aarch64, the NVX CFFI and PyPy-specific build handling work, and the release and inventory entries are updated.