Add compatibility with std::pmr

[ktf]

The move assign operator is deleted is std::pmr::polymorphic_allocator so we cannot iterate
anymore at runtime on the headers in the stack and move them.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 855ea9c at 2025-06-10 15:53:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/14379-slc9_x86-64/0/Detectors/TPC/workflow/src/ClusterDecoderRawSpec.cxx:89:20: error: no matching function for call to 'swap(o2::header::Stack&, o2::header::Stack&)'
/sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/include/c++/14.2.0/type_traits:138:11: error: no type named 'type' in 'struct std::enable_if<false, void>'
/sw/SOURCES/O2/14379-slc9_x86-64/0/Detectors/TPC/workflow/src/ClusterDecoderRawSpec.cxx:98:18: error: no matching function for call to 'swap(o2::header::Stack&, o2::header::Stack&)'
ninja: build stopped: subcommand failed.

Full log here.

[ktf]

@knopers8 @Barthelemy any objections to this? I assume we will never have more than 8 headers to copy...

[knopers8]

@ktf here, you used 5, but the code below supports up to 8. A typo?

[ktf]

Yes. It used to be 5 and I then changed it to 8. Will fix this.