[diskann-wide] Add byte-vector reinterpretations for u32x8 and u64x8

[juchen-ms (partychen)]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies? No.
• Does this PR modify any existing APIs? Adds trait implementations only.
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation? No separate documentation changes are needed.

Reference Issues/PRs

Extracted from #1394 to land the reusable diskann-wide support independently.

What does this implement/fix? Briefly explain your changes.

Adds bit-preserving SIMD reinterpretations from word lanes to byte lanes:

• V3: u32x8 → u8x32 and i8x32.
• V4: u64x8 → u8x64 and i8x64.
• Matching little-endian Emulated implementations for all four conversions.

Tests follow the existing conversion-test macro and architecture-check conventions. They compare raw bytes using distinct byte positions, signed-byte boundary patterns, and the existing seeded test driver.

The change is limited to diskann-wide. No MaxSim consumers, custom intrinsics, or unrelated refactors are included.

Any other comments?

Local validation passed:

• cargo test -p diskann-wide
• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo +nightly miri test -p diskann-wide --lib reinterpret_

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementations preserve register bits and include targeted byte-order validation across native and emulated paths.

Review effort: Balanced
Findings: None

What changed in this PR

Adds tested SIMD bit reinterpretation support for additional x86 and emulated vector widths.

Changes:

• Adds reusable reinterpretation tests.
• Supports u32x8 and u64x8 reinterpretation into signed and unsigned byte vectors.
• Adds corresponding emulated implementations.

File	Description
diskann-wide/​src/​test_utils/​ops.rs	Adds the reinterpretation test macro.
diskann-wide/​src/​emulated.rs	Adds emulated reinterpretations and tests.
diskann-wide/​src/​arch/​x86_64/​v3/​conversion.rs	Adds 256-bit reinterpretations and tests.
diskann-wide/​src/​arch/​x86_64/​v4/​conversion.rs	Adds 512-bit reinterpretations and tests.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.87%. Comparing base (fcf9053) to head (839cb7e).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-wide/src/arch/x86_64/v4/conversion.rs	0.00%	6 Missing ⚠️

❌ Your patch status has failed because the patch coverage (81.25%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1445      +/-   ##
==========================================
+ Coverage   90.58%   90.87%   +0.29%     
==========================================
  Files         568      579      +11     
  Lines      112680   114587    +1907     
==========================================
+ Hits       102069   104132    +2063     
+ Misses      10611    10455     -156     

Flag	Coverage Δ
miri	90.87% <81.25%> (+0.29%)	⬆️
unittests	90.64% <81.25%> (+0.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-wide/src/arch/x86_64/v3/conversion.rs	100.00% <100.00%> (+31.57%)	⬆️
diskann-wide/src/emulated.rs	100.00% <ø> (+1.68%)	⬆️
diskann-wide/src/test_utils/ops.rs	98.46% <100.00%> (+0.04%)	⬆️
diskann-wide/src/arch/x86_64/v4/conversion.rs	0.00% <0.00%> (ø)

... and 38 files with indirect coverage changes

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