Parent tracking issue: #1950

The x86 ASan CI already reports several UBSan runtime errors, but the job still passes because the current sanitizer configuration allows UBSan recovery.

Related x86 CI logs:

• ASan Build X86: https://github.com/antgroup/vsag/actions/runs/25100797892/job/73549271518
• Test X86 Unittests: https://github.com/antgroup/vsag/actions/runs/25100797892/job/73553696204
• Test X86 Functests: https://github.com/antgroup/vsag/actions/runs/25100797892/job/73553696201

The build log shows GCC 11.4.0 and Compiling with AddressSanitizer and UndefinedBehaviorSanitizer, so these reports are not caused by macOS-only sanitizer behavior.

Observed x86 UBSan reports include:

• src/algorithm/hnswlib/hnswalg.cpp:319: store to misaligned address for linklistsizeint.
• src/algorithm/hnswlib/hnswalg.cpp:342: store to misaligned address for linklistsizeint.
• src/algorithm/hnswlib/hnswalg.h:422: load of misaligned address for unsigned short.
• src/algorithm/hnswlib/hnswalg.h:427: store to misaligned address for unsigned short.
• src/impl/bitset/fast_bitset.cpp:237: null pointer passed to memcpy.
• src/algorithm/inner_index_interface.cpp:200: null pointer passed to memcpy.
• src/datacell/sparse_term_datacell.cpp:150: reference binding to null pointer.

Why CI still passes:

The current ASan build uses -fsanitize=address,undefined, but it does not make UBSan reports fatal. There is no UBSAN_OPTIONS=halt_on_error=1, and the build does not use -fno-sanitize-recover=undefined. As a result, UBSan prints runtime error diagnostics and the test process can still exit with code 0.

Sub-issues:

• Fix HNSW unaligned memory access reported by UBSan #1962
• Fix FastBitset zero-length memcpy with null data #1963
• Guard InnerIndexInterface deserialize reads against empty binary data #1964
• Handle null term id lists in SparseTermDataCell search #1965

Related macOS-observed UBSan follow-up:

• Handle NaN scalar quantizer encoding before uint8 casts #1966

Suggested plan:

• Fix each UB class in focused PRs.
• Start with the deterministic x86 reports above, since they are already visible in current CI logs.
• After known UBSan reports are fixed, consider making sanitizer CI stricter by setting UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 or adding -fno-sanitize-recover=undefined.

Non-goals:

• Do not mix these fixes into the macOS CI/build PR.
• Do not enable fatal UBSan mode before the known reports are fixed, otherwise existing CI will fail immediately.