v1.5.2

Packaging patch that unbreaks the musllinux (Alpine) Python wheels and adds
guardrails so the same regression cannot ship again. No source or API
changes; glibc, macOS, FreeBSD, and Windows builds are byte-for-byte
unaffected.

Bug Fixes

• regopy musllinux wheels for 1.4.0 through 1.5.1 failed to load on Alpine
  with Error relocating librego_shared.so: __cxa_thread_atexit_impl: symbol not found. On non-glibc Linux, the snmalloc thread-teardown mode is now
  forced to PTHREAD_DESTRUCTORS, which does not depend on the glibc-only
  __cxa_thread_atexit_impl symbol. The resulting library loads cleanly on
  Alpine and other musl-based distributions.

Build / Packaging

• Root CMakeLists.txt probes the target libc via <features.h> and
  exposes the result as REGOCPP_HAVE_GLIBC. The probe runs on every
  configure so a toolchain switch in an existing build directory is not
  answered from a stale cache.
• src/CMakeLists.txt adds a POST_BUILD symbol assertion on
  librego_shared.so when building for a non-glibc Linux, driven by a new
  cmake/check_forbidden_symbols.cmake helper. A future dependency that
  reintroduces a glibc-only undefined reference fails the build rather than
  the end user's dlopen.
• The SNMALLOC_CLEANUP override transparently overwrites a stale
  CXX11_THREAD_ATEXIT_DIRECT cache value from an earlier configure, so
  developers iterating in the same build directory do not need to wipe it.

CI

• pr_gate.yml gains a linux-musllinux-load job that rebuilds
  librego_shared.so inside quay.io/pypa/musllinux_1_2_x86_64 and
  dlopens it in a plain python:3.13-alpine, catching any glibc-only
  undefined symbol on every PR.
• build_wheels.yml gains a smoke_test_musllinux job that installs each
  built musllinux wheel by explicit path (with a wheel-count assertion) in
  the matching python:X-alpine image and exercises Interpreter().query
  from both the main thread and a worker thread, verifying that snmalloc's
  pthread-key teardown runs cleanly.