Add Neon inner-product kernel for USlice<2> with spherical wiring

[pfoxARM]

What does this implement?

Adds Neon Inner product kernel USlice<2> * USlice<2> into diskann-quantization/src/bits/distances.rs
Adds 2-bit spherical wiring into diskann-quantization/src/spherical
Updates unit test bounds

[Copilot]

Pull request overview

This PR extends DiskANN’s quantized distance infrastructure by adding an AArch64/Neon-optimized inner-product kernel for 2-bit unsigned bit-slices and wiring it into the spherical quantizer dispatch so 2-bit spherical distances can run on Neon instead of retargeting to scalar.

Changes:

• Added a Neon Target2 implementation for InnerProduct over USlice<2> × USlice<2> and updated the kernel support table and test bounds.
• Updated spherical dispatch_map! wiring to use Neon directly for AsData<2> and AsQuery<2> (no downcast/retarget).
• Added explicit AArch64 codegen instantiations for the 2-bit spherical inner-product paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
diskann-quantization/src/spherical/iface.rs	Enables Neon dispatch for 2-bit spherical data/query layouts by removing the downcast/retarget.
diskann-quantization/src/spherical/__codegen/aarch64.rs	Adds explicit codegen instantiations for the 2-bit Neon spherical inner-product paths.
diskann-quantization/src/bits/distances.rs	Implements the 2-bit Neon inner-product kernel, updates retargeting, and adjusts test bounds/documentation.

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[Mark Hildebrand (hildebrandmw)]

Thanks!

[pfoxARM]

Should of fixed cargo-fmt errors now.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (fabcb9b) to head (a600f9c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1363      +/-   ##
==========================================
- Coverage   91.55%   91.54%   -0.01%     
==========================================
  Files         521      521              
  Lines      100371   100302      -69     
==========================================
- Hits        91895    91825      -70     
- Misses       8476     8477       +1     

Flag	Coverage Δ
miri	91.54% <100.00%> (-0.01%)	⬇️
unittests	91.23% <100.00%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
diskann-quantization/src/bits/distances.rs	99.59% <100.00%> (ø)
diskann-quantization/src/spherical/iface.rs	90.19% <ø> (ø)

... and 16 files with indirect coverage changes

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