Add Neon Inner product u4*u4 kernel

[pfoxARM]

• Does this PR have a descriptive title that could go in our release notes? Yes
• Does this PR add any new dependencies? No
• Does this PR modify any existing APIs? No
• Is the change to the API backwards compatible? No
• Should this result in any changes to our documentation, either updating existing docs or adding new ones? No

What does this implement?

Exclusive aarch64 USlice4 * USlice 4 Inner Product kernel using Neon and dotprod. We also add quantization instantiation for spherical quantization.

Any other comments?

We use dot_simd() heavily in this kernel, for performance we rely on dotprod feature.

[Copilot]

Pull request overview

This PR adds an AArch64-specific Neon+dotprod inner-product kernel for 4-bit (USlice<4> × USlice<4>) paths in spherical quantization, and wires it into the spherical quantizer’s architecture dispatch so the Neon implementation can be selected where available.

Changes:

• Add aarch64 spherical __codegen instantiations for the 4-bit Neon inner-product paths.
• Enable Neon dispatch for spherical quantization AsData<4> and AsQuery<4> without downcasting to Scalar.
• Implement an AArch64 Neon InnerProduct kernel for USlice<4> × USlice<4> and adjust retargeting to avoid overlap.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
diskann-quantization/src/spherical/iface.rs	Updates dispatch mapping so 4-bit spherical paths can use Neon directly (no downcast).
diskann-quantization/src/spherical/__codegen/mod.rs	Adds an AArch64 codegen module behind cfg(target_arch = "aarch64").
diskann-quantization/src/spherical/__codegen/aarch64.rs	New AArch64 instantiation helpers for the 4-bit Neon inner-product distance computer.
diskann-quantization/src/bits/distances.rs	Adds the Neon USlice<4> × USlice<4> inner-product implementation and updates retargeting accordingly.

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[pfoxARM]

@microsoft-github-policy-service agree company="Arm"

[Mark Hildebrand (hildebrandmw)]

Thanks! Looks good to me - great to start having Neon kernels!

Outside of the small tweak to the test bounds, please start a new aarch64.rs file in diskann-quantization/src/__codegen.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.55%. Comparing base (158126e) to head (a654681).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1353      +/-   ##
==========================================
- Coverage   91.55%   91.55%   -0.01%     
==========================================
  Files         521      521              
  Lines      100371   100371              
==========================================
- Hits        91898    91895       -3     
- Misses       8473     8476       +3     

Flag	Coverage Δ
miri	91.55% <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 4 files with indirect coverage changes

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