[s2x]

What changed

Removes musl (Alpine) support entirely: the build-piper-musl job in ci.yml and
release.yml (with its entries in the ci-ok and publish needs: lists), the two musl
release assets, the alpine container prose in docs/release-workflow.md, the README
instructions to install ONNX Runtime from the Alpine package, and the musl job in AGENTS.md.
The README now states the supported platform (Linux x86_64 with glibc) and CHANGELOG.md gets
a Removed entry with the reasons and what an Alpine user should do instead.

Why: libpiper links ONNX Runtime and Microsoft publishes no musl build of it, so there is
nothing on Alpine to link libpiper against except a package from Alpine's own repository. The
musl archives we shipped never worked anyway: libonnxruntime-linux-musl-x86_64.tar.gz up to
v0.2.0 holds a glibc library (GLIBC_2.2.5 to GLIBC_2.27), which the musl loader cannot
resolve.

Carried over from #22, which solved most of #21 with the rejected keep-musl approach: the
piper1-gpl submodule bump v1.4.1 -> v1.8.0 and the espeak-ng-data -> share/espeak-ng-data
install path fix (the test-integration failure on Dependabot PR #3). The Alpine-package part
of #22 is not carried over.

Licensing is untouched: MIT for the PHP code, GPL-3.0 for libpiper and the espeak-ng data.

Closes #26

How it was tested

• bin/lint.sh (composer validate/audit, php-cs-fixer, PHPStan, Rector, shellcheck) and
  vendor/bin/phpunit --testsuite unit (14 tests) in the worktree.
• The FFI cdef in src/PiperTTS.php was checked against the new
  piper1-gpl/libpiper/include/piper.h: piper_audio_chunk and piper_synthesize_options
  keep their field order and types, piper_create is unchanged, and the new functions
  (piper_create_with_options, piper_version) are additive, so the public PHP API does not
  change.
• End to end on glibc on a Linux x86_64 host (Ubuntu, cmake 4.2.3, PHP 8.5.4): make build-libs
  (libpiper v1.8.0 + ONNX Runtime 1.22.0, libs/espeak-ng-data from the new share/ path),
  make test-model, unit suite 14/14 and vendor/bin/phpunit --group integration 10/10.
• Both workflow files parse as YAML and every name in every needs: exists as a job;
  ci-ok lists every other job in ci.yml. No test reads .github/workflows/, so no test
  asserted the old job list.

Checklist

• Tests added or updated, and all checks pass
• CHANGELOG.md updated under [Unreleased]
• Documentation updated
• Everything is written in English
• The PR title is a Conventional Commit (feat: ..., fix: ...)