Fix alignment for in-memory allreduce buffer

[RAMitchell]

Summary

Fix alignment for the shared buffer used by in-memory collective allreduce.

The previous implementation stored bytes in a std::string and then reinterpreted the buffer as typed data for reduction. That provides contiguous storage, but not the alignment guarantees required for typed access. This change switches the shared buffer to aligned storage while preserving the existing byte-oriented behavior for the in-memory collective path.

No intended user-facing changes.

Verification

Built src/collective/in_memory_handler.cc in the existing build/ tree.

[Copilot]

Pull request overview

Replaces std::string with a custom AlignedByteBuffer backed by std::max_align_t storage to guarantee proper alignment for typed reinterpret_casts during in-memory allreduce.

Changes:

• Introduced AlignedByteBuffer class providing aligned byte storage with a std::string-like interface
• Updated all functor signatures and buffer operations to use AlignedByteBuffer
• Added alignment/size checks in the allreduce path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/collective/in_memory_handler.h	Added AlignedByteBuffer class; changed buffer_ member type
src/collective/in_memory_handler.cc	Updated functors to use AlignedByteBuffer; added checks; fixed Accumulate signature

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