Add musllinux wheel packaging for Alpine

[jtwigg-luma]

Why

Upstream contentauth/c2pa-python only publishes manylinux (glibc) wheels. Alpine (musl) cannot load those wheels; the sdist pulls gnu natives that fail at dlopen; compiling Rust inside every image build is too slow.

This fork ships musllinux wheels so Alpine can import c2pa without a per-image Rust compile. The Python API is unchanged. macOS and glibc Linux keep using PyPI.

Divergence is kept small: one build script, one workflow, MUSLLINUX.md, and disabled unused Adobe Actions so musl tags do not fan out into their Build/PyPI/Pages/Jira matrix.

What

• scripts/build_musllinux_wheel.sh — build musllinux_1_2_x86_64 in python:3.14-alpine
• .github/workflows/build-musllinux-wheel.yml — CI + GitHub Release on v*-musl* tags
• MUSLLINUX.md — why this fork exists + how to upgrade
• Disable unused upstream Actions (stub workflow_dispatch only)

Consume

dependencies = [
  "c2pa-python==0.37.1",
]

[tool.uv]
find-links = [
  "https://github.com/<org>/<repo>/releases/expanded_assets/v0.37.1-musl.1",
]

Do not pin a direct musllinux wheel URL (forces one platform everywhere).

Test plan

• Wheel installs in python:3.14-alpine → import c2pa + Context() OK
• Release v0.37.1-musl.1 published with wheel asset
• CI musllinux workflow succeeds on workflow_dispatch
• Approve + squash-merge (org ruleset requires 1 review)

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