fix: avoid unaligned hnsw link access

[jac0626]

Summary

Fixes #1962.

This PR removes undefined behavior from HNSW packed link-list access by replacing direct typed loads/stores on potentially unaligned graph memory with small inline std::memcpy accessors.

Root Cause

HNSW stores link-list counts and neighbor ids in a packed byte layout. Some access paths cast those byte addresses to linklistsizeint*, unsigned short*, or integer pointers and dereference them directly. UBSan reports this as misaligned load/store even when the code happens to work on x86 hardware.

Changes

• Use inline unaligned scalar load/store helpers for list count access.
• Use the same helpers for direct neighbor id reads/writes on packed link-list memory.
• Parse serialized HNSW header fields from char buffers via std::memcpy instead of typed pointer casts.
• Preserve the existing in-memory and serialized layout.

Validation

• Ran clang-format 15 on the changed HNSW files.
• Ran git diff --check.
• Did not add new tests; the existing ASan/UBSan functest path already exposes this issue, so CI should validate that the HNSW unaligned runtime errors disappear.

[mergify]

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[gemini-code-assist]

Code Review

This pull request refactors memory access within the HNSW algorithm to safely handle unaligned data by replacing direct pointer casting and arithmetic with specialized helper functions. New templates readUnaligned and writeUnaligned utilize std::memcpy to prevent undefined behavior, and higher-level accessors like getLinkAt and setLinkAt have been implemented to standardize link list manipulations. Feedback suggests strengthening the safety of the unaligned access templates by adding a static_assert to verify that the types being copied are trivially copyable.

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