Build Linux release wheels for x86_64 only

With the manylinux_2_28 change in place all four x86_64 wheels built,
and the release run moved on to i686, where cp312 failed the same way:

    Host machine cpu: i686
    C compiler ... (gcc 10.2.1 ...)
    ../meson.build:25:4: ERROR: Problem encountered: NumPy requires GCC >= 10.3

That one cannot be fixed by changing images. NumPy publishes no i686
wheels for any 2.x release, so pip can only reach NumPy through its
sdist, and the manylinux2014_i686 image's GCC 10.2.1 is below NumPy's
floor. Restrict Linux builds to x86_64; cmac has never published an
i686 wheel.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Robert Jackson

pyproject.toml

@@ -82,6 +82,12 @@ build_ext = "_cmac_build.build_ext"
 skip = "pp*"
 
 [tool.cibuildwheel.linux]
+# NumPy publishes no i686 wheels for any 2.x release, so a 32-bit build can
+# only get NumPy by compiling it from source in the manylinux2014_i686 image,
+# whose GCC 10.2.1 NumPy's own meson rejects. 64-bit only; no i686 wheel has
+# ever been published for this project anyway.
+archs = "x86_64"
+
 # cibuildwheel defaults to the manylinux2014 image (glibc 2.17, GCC 10.2.1).
 # NumPy >= 2.3 publishes only manylinux_2_28 wheels, so nothing installable is
 # available in that image; pip falls back to the NumPy sdist and meson refuses