Test by-element sdot/udot codegen on ARM

[alexreinking]

Adds correctness_arm_dot_product_by_element, which locks in by-element sdot/udot codegen on ARM (sdot vD.4s, vN.16b, vM.4b[lane], the vdotq_laneq_s32 instruction).

The test compiles an 8-row gemv with weights interleaved 4 rows at a time in 4-byte groups, as in ggml's q4_0_4x4 and KleidiAI's dotprod kernels. It stages 16 activation bytes in a register and unrolls 4 groups.

It requires the by-element form on all four lanes, for sdot and udot, with the broadcast operand on either side of the multiply. On an AArch64 host with dotprod it also checks values against a scalar reference.

This relies on CodeGen_ARM moving the broadcast to the second operand of dot_product (since 8aa778c), which had no test. With that swap removed, the test fails (found 0 lanes).

Test-only; no compiler changes.

Validation on macOS arm64: ctest --test-dir build/macOS -R correctness_arm_dot_product_by_element passes, and fails with the swap removed.

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Authored by GitHub Copilot

[alexreinking]

I'm adding this test because Copilot spent well over an hour discovering this recipe even after searching the repo for key terms.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.05%. Comparing base (ee9f17a) to head (1db2602).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9544   +/-   ##
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  Coverage   71.05%   71.05%           
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  Files         262      262           
  Lines       81084    81084           
  Branches    19770    19770           
=======================================
+ Hits        57615    57618    +3     
+ Misses      17502    17488   -14     
- Partials     5967     5978   +11     

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