feat: add musl (Alpine Linux) support and packaging

[clemperorpenguin]

This PR adds support for building and running lemonade on musl-based Linux (Alpine), covering both the C++ server and native packaging. llama.cpp (CPU + Vulkan) is verified working on Alpine; the remaining backends are in progress.

What's included:

• Compile-time libc detection (platform.h): defines LEMON_LINUX_MUSL on linux && !GLIBC, which drives backend asset selection.
• Per-backend musl asset selection (llama.cpp, stable-diffusion.cpp, whisper.cpp, Kokoro, moonshine): musl builds fetch dedicated …-musl-… assets, never a glibc binary.
• Alpine packaging (packaging/alpine/): APKBUILD (OpenRC instead of systemd), install scripts, build-apk.sh; adds onnxruntime runtime dep for the ONNX audio backends.
• CI (alpine_apk_build.yml): builds + smoke-tests the .apk for x86_64 and aarch64.
• setup.sh: apk branches at each package-manager choke point.
• Dockerfile.alpine and a LEMONADE_BACKEND_REPO_OWNER testing hook (no-op when unset).

Depends on:
Companion changes to the backend release repos publish the …-musl-… assets (separate PRs). backend_versions.json intentionally unchanged.

Status:
llama.cpp CPU + Vulkan verified on Alpine; sd.cpp / whisper / Kokoro / moonshine in progress.

Cross-platform safety:
All C++ changes are #ifdef LEMON_LINUX_MUSL-guarded; shell changes are apk-only. glibc / Windows / macOS unchanged.

[clemperorpenguin]

PR depends on merges:
lemonade-sdk/llama.cpp#22
lemonade-sdk/stable-diffusion.cpp#16
lemonade-sdk/whisper.cpp-rocm#7
lemonade-sdk/moonshine-server-rocm#1
lemonade-sdk/Kokoros#5

Set LEMONADE_BACKEND_REPO_OWNER=clemperorpenguin for testing :)

Note: I can't get the whisper CI to pass, not sure why. Windows NPU builds for a day and then cancels the job.

[fl0rianr]

Thanks for pushing Alpine/musl support — the APK/Docker direction is good and the llama.cpp CPU/Vulkan smoke coverage is useful. I don’t think this is merge-ready yet.

Blocking issues:

• Dockerfile.alpine bakes LEMONADE_BACKEND_REPO_OWNER=clemperorpenguin into the runtime image. That makes official images fetch backend binaries from a personal fork by default. Please default this to empty and only pass it explicitly in temporary CI/testing, or remove it once the companion asset PRs have landed.

• The OpenRC service does not export XDG_RUNTIME_DIR or RUNTIME_DIRECTORY, while the Linux runtime-dir resolver requires one of them. The APK CI only tests direct lemond startup with a manually exported XDG_RUNTIME_DIR, so it does not cover the documented rc-service lemonade-server start path.

• musl hosts can still select/download glibc Linux assets for some backends, especially llamacpp CUDA/ROCm and whispercpp ROCm. Please either mark those variants unsupported on musl or remap them consistently, and add install-param tests that assert musl never resolves Ubuntu/glibc asset names.

• The PR depends on several unmerged backend release PRs for the musl assets. Please merge/release those first or keep this PR non-mergeable until the assets are available from official repos.

Also please reconcile the aarch64 descriptor support with the newly added aarch64 asset names and update the Dockerfile/README wording around Moonshine.

[fl0rianr]

Thanks for the updates — the main blockers from my previous review are addressed now.

Remaining open points:

• The companion backend asset PRs still need to land I guess you have this in plan
• Please remove the temporary fork build-arg from the Alpine Docker smoke workflow once the companion assets are upstream.

After those move forward, I’m approving this.

[clemperorpenguin]

I've thought about this more, it will either add tech debt to lemonade or to ggml, and I do not know that there is much demand for an Alpine package at this point - ROCm is not even fully packaged on Alpine yet afaik. Further, with custom backends being discussed/upcoming, and recent development of https://github.com/pg83/solo , I'm not sure if native musl support makes sense at this time without a way to run the glibc binaries (like with solo). Closing for now to refocus on other priorities.