fix: validate unaligned offset buffers safely

[efegokdemir]

Summary

Fixes #945 by making validation of Arrow offset buffers safe for unaligned C Data Interface buffers.

Changes

• Add byte-based memcpy helpers for reading 32-bit and 64-bit buffer values.
• Use the helpers for default and full offset validation, including string, list, and large variants.
• Add regression coverage for unaligned string and large-string offset buffers.

Testing

• Upstream reproducer with Clang UBSan: reproduced misaligned-load failure.
• Patched bundle with Clang UBSan: passed; ArrowArrayViewValidate: 0 with no UBSan diagnostics.
• cc -std=c99 -Wall -Wextra -Werror -I/tmp/nanoarrow945-dist-fixed/include -Isrc -fsyntax-only src/nanoarrow/common/array.c: passed.
• git diff --check: passed.
• Full CMake/GoogleTest suite: not run because CMake and GoogleTest are not installed in the local environment.

Notes

The change keeps the existing validation results and avoids typed dereferences of externally supplied offset buffers. AI assistance was used during investigation and implementation; the submitter reviewed the changed lines and validation results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.19%. Comparing base (ec8a58c) to head (46f59e5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #946   +/-   ##
=======================================
  Coverage   78.19%   78.19%           
=======================================
  Files         106      106           
  Lines       16843    16845    +2     
  Branches     1985     1985           
=======================================
+ Hits        13170    13172    +2     
  Misses       2439     2439           
  Partials     1234     1234           

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[paleolimbot]

Thank you!

I'll open a follow up to try to integrate these more...there are definitely a few more places we're reading unaligned buffers.