Add native SIMD testing and benchmark module

[r-devulap]

This pull request adds comprehensive native C++ unit tests and benchmarks for the SIMD kernel dispatcher and vector similarity functions, and integrates them into the build and CI pipeline. The most important changes are:

Testing and Benchmarking Enhancements:

• Adds a new test_simd_kernels executable using Google Test, with tests for ISA detection and SIMD kernel correctness, and integrates it into the Meson build and GitHub Actions CI workflow.
• Adds a new bench_simd_kernels executable using Google Benchmark to micro-benchmark cosine_f32, dot_product_f32, and euclidean_f32 functions at realistic embedding dimensions.

Build System and Code Organization:

• Moves the core SIMD kernel and dispatcher source files into a src/ subdirectory and updates the Meson build rules and include paths accordingly. The native code layout has been reorganized in preparation for modularizing it into a standalone library, improving structure and maintainability.

CI Improvements:

• Updates the GitHub Actions workflow to install libgtest-dev, build, and run the new native C++ tests for all relevant ISA tiers, ensuring dispatcher correctness across hardware.

[github-actions]

Before you submit for review:

• Does your PR follow guidelines from CONTRIBUTIONS.md?
• Did you summarize what this PR does clearly and concisely?
• Did you include performance data for changes which may be performance impacting?
• Did you include useful docs for any user-facing changes or features?
• Did you include useful javadocs for developer oriented changes, explaining new concepts or key changes?
• Did you rebase your branch onto the latest main for regression testing and PR submission?
• Did you trigger regression testing via Run Bench Main and review results?
• Did you adhere to the code formatting guidelines (TBD)
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• Did you add documentation for this feature to the release notes directory?

If you did not complete any of these, then please explain below.

[MarkWolters]

code looks good, very thorough. I think this will make a good addition to our test suite. Please just update the comments in bench_similarity_f32.cpp and/or add a README and I think it's good to go.

[MarkWolters]

I think this looks like it is probably fine, but for future reference it would be easier to verify if this was a branch and not coming from a fork. Since it's a fork I can't just run the gha and verify the results, which would make it easier to be comfortable with approval. Not a showstopper but something to keep in mind for future PRs.

[MarkWolters]

This doesn't work for me. I cloned the fork but this command fails with the following

ubuntu@ip-172-31-6-106:~/jvector/jvector-native/src/main/native$ meson setup build && ninja -C build bench_simd_kernels
Directory already configured.

Just run your build command (e.g. ninja) and Meson will regenerate as necessary.
Run "meson setup --reconfigure to force Meson to regenerate.

If build failures persist, run "meson setup --wipe" to rebuild from scratch
using the same options as passed when configuring the build.
ninja: Entering directory `build'
ninja: error: unknown target 'bench_simd_kernels'
ubuntu@ip-172-31-6-106:~/jvector/jvector-native/src/main/native$ git status
On branch simd-testing
Your branch is up to date with 'origin/simd-testing'.

ubuntu@ip-172-31-6-106:~/jvector/jvector-native/src/main/native$ meson setup build --wipe
The Meson build system
Version: 1.3.2
Source dir: /home/ubuntu/jvector/jvector-native/src/main/native
Build dir: /home/ubuntu/jvector/jvector-native/src/main/native/build
Build type: native build
Project name: jvector_simd_kernels
Project version: 0.1.0
C++ compiler for the host machine: c++ (gcc 13.3.0 "c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0")
C++ linker for the host machine: c++ ld.bfd 2.42
Host machine cpu family: x86_64
Host machine cpu: x86_64
Found pkg-config: YES (/usr/bin/pkg-config) 1.8.1
Run-time dependency gtest_main found: YES 1.14.0
Found CMake: /usr/bin/cmake (3.28.3)
Run-time dependency benchmark found: NO (tried pkgconfig and cmake)
Build targets in project: 7

Found ninja-1.11.1 at /usr/bin/ninja
ubuntu@ip-172-31-6-106:~/jvector/jvector-native/src/main/native$ ninja -C build bench_simd_kernels
ninja: Entering directory `build'
ninja: error: unknown target 'bench_simd_kernels'

[MarkWolters]

Oh it's test_simd_kernels, not bench_simd_kernels. Just needs an updated comment(s). Might be worth including a README that lists the steps, I had to walk through the github workflow to understand what to do.

[MarkWolters]

I think you want set -x here, set +x turns off debugging