* fix: detect musl libc when downloading python-build-standalone
`platform.libc_ver()` only ever reports glibc and returns `("", "")` on
musl systems, so `"musl" in platform.libc_ver()` was never true and musl
hosts (e.g. Alpine) always selected the `gnu` python-build-standalone
asset, which won't run.
Detect musl by shelling out to `ldd --version`, which prints "musl libc"
on musl-based systems. This probes the actual system loader rather than
the host Python's linkage, so it stays correct even when cibuildwheel
runs under a relocatable/statically-linked interpreter. Falls back to
glibc when `ldd` is unavailable.
Assisted-by: ClaudeCode:claude-opus-4.8
* test: add type annotations to satisfy mypy and ruff
Assisted-by: ClaudeCode:claude-opus-4.8
