2025.10.06 FTBFS on clang-i586: static assertion failed due to requirement 'alignof...'

Version: folly 2025.10.06.00

Using clang-21.1.2 for i586-linux-gnu from openSUSE Tumbleweed/Build Service, one observes:

```
[ 16s] /usr/bin/clang++ -DBOOST_ATOMIC_DYN_LINK -DBOOST_ATOMIC_NO_LIB -DBOOST_CHRONO_DYN_LINK -DBOOST_CHRONO_NO_LIB -DBOOST_CONTAINER_DYN_LINK -DBOOST_CONTAINER_NO_LIB -DBOOST_CONTEXT_DYN_LINK -DBOOST_CONTEXT_NO_LIB -DBOOST_DATE_TIME_DYN_LINK -DBOOST_DATE_TIME_NO_LIB -DBOOST_FILESYSTEM_DYN_LINK -DBOOST_FILESYSTEM_NO_LIB -DBOOST_PROGRAM_OPTIONS_DYN_LINK -DBOOST_PROGRAM_OPTIONS_NO_LIB -DBOOST_REGEX_DYN_LINK -DBOOST_REGEX_NO_LIB -DBOOST_THREAD_DYN_LINK -DBOOST_THREAD_NO_LIB -DFMT_SHARED -DFOLLY_XLOG_STRIP_PREFIXES=\"/home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00:/home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00/build\" -DGFLAGS_IS_A_DLL=0 -DGLOG_USE_GLOG_EXPORT=1 -DGOOGLE_GLOG_IS_A_DLL=1 -D_GNU_SOURCE -D_REENTRANT -I/home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00 -I/home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00/build -fomit-frame-pointer -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -g -ffat-lto-objects -fPIC -O2 -g -DNDEBUG -std=gnu++20 -fPIC -g -finput-charset=UTF-8 -fsigned-char -Wall -Wno-deprecated -Wno-deprecated-declarations -Wno-sign-compare -Wno-unused -Wuninitialized -Wunused-label -Wunused-result -Wno-noexcept-type -Wno-nullability-completeness -Wno-inconsistent-missing-override -faligned-new -fopenmp -MD -MT CMakeFiles/folly_base.dir/folly/CancellationToken.cpp.o -MF CMakeFiles/folly_base.dir/folly/CancellationToken.cpp.o.d -o CMakeFiles/folly_base.dir/folly/CancellationToken.cpp.o -c /home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00/folly/CancellationToken.cpp
[ 16s] /home/abuild/rpmbuild/BUILD/folly-2025.10.06.00-build/folly-2025.10.06.00/folly/CancellationToken.cpp:307:17: error: static assertion failed due to requirement 'alignof(folly::detail::MergingCancellationState) <= alignof(max_align_t)'
[ 16s] 307 | static_assert(alignof(MergingCancellationState) <= alignof(std::max_align_t));
[ 16s] | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Reproduce the folly 2025.10.06.00 build with clang-21.1.2 for i586-linux-gnu and inspect folly/CancellationToken.cpp around line 307. Determine why MergingCancellationState exceeds std::max_align_t on this target; done when the i586 build completes without the reported static assertion.

Written by the indexing model from the issue text.