PiPNN 1/6: add numerical kernels

[SeliMeli]

Adds the numerical kernels required by later PiPNN graph-construction layers.

Review key points

• Fallible lower-triangular AAT plus partition and leaf top-k kernels.
• Architecture-generic diskann-wide dispatch; no ISA detection or intrinsics in PiPNN.
• Differential boundary tests, x86 fallback/AArch64 builds, SDE coverage, and Criterion workloads.

Stack 1/6 → #1288

[Copilot]

Pull request overview

This PR adds the first set of PiPNN “kernel” building blocks to the DiskANN Rust workspace: SIMD-accelerated top‑k selection for partition assignment and leaf neighbor selection, along with supporting SIMD division and a new lower-triangular A·Aᵀ helper in diskann-linalg.

Changes:

• Add a new diskann-pipnn crate with partition_kernel and leaf_kernel implementations plus extensive correctness tests and Criterion benchmarks.
• Extend diskann-wide to support Div on relevant f32 SIMD types (native, doubled, and scalar/emulated) and add a corresponding division test macro.
• Add diskann_linalg::sgemm_aat_lower (lower-triangle-only AAT) and wire new crate/tests/CI/mutants exclusions into the workspace.

Reviewed changes

Copilot reviewed 26 out of 27 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
diskann-wide/src/test_utils/ops.rs	Adds test_div! macro to validate lane-wise SIMD division correctness.
diskann-wide/src/emulated.rs	Adds Div for scalar/emulated Emulated<f32, N, A> to support division in scalar dispatch.
diskann-wide/src/doubled.rs	Adds Div for Doubled<T> to support composite SIMD widths.
diskann-wide/src/arch/x86_64/v4/f32x8_.rs	Adds AVX Div op mapping + division tests.
diskann-wide/src/arch/x86_64/v4/f32x4_.rs	Adds SSE Div op mapping + division tests.
diskann-wide/src/arch/x86_64/v4/f32x16_.rs	Adds AVX-512 Div op mapping + division tests.
diskann-wide/src/arch/x86_64/v3/f32x8_.rs	Adds AVX Div op mapping + division tests for V3.
diskann-wide/src/arch/x86_64/v3/f32x4_.rs	Adds SSE Div op mapping + division tests for V3.
diskann-wide/src/arch/x86_64/v3/f32x16_.rs	Adds division tests for the f32x16 V3 path (likely via doubled composition).
diskann-wide/src/arch/aarch64/f32x4_.rs	Adds Neon Div op mapping + division tests.
diskann-wide/src/arch/aarch64/f32x2_.rs	Adds Neon Div op mapping + division tests.
diskann-pipnn/tests/partition_kernel.rs	New integration tests for partition top‑k dispatch correctness and edge cases.
diskann-pipnn/tests/leaf_kernel.rs	New integration tests for leaf neighbor top‑k dispatch correctness and edge cases.
diskann-pipnn/src/partition_kernel/tests.rs	New unit tests comparing scalar reference vs runtime dispatch and metric contracts.
diskann-pipnn/src/partition_kernel.rs	New partition-assignment distance + top‑k kernel with validation and SIMD dispatch.
diskann-pipnn/src/lib.rs	New crate root exporting PiPNN kernel modules.
diskann-pipnn/src/leaf_kernel/tests.rs	New unit tests for scalar reference parity and workspace behavior.
diskann-pipnn/src/leaf_kernel.rs	New fused lower-triangle leaf neighbor kernel with SIMD dispatch and workspace support.
diskann-pipnn/Cargo.toml	Defines new diskann-pipnn crate, dev-deps, and benches.
diskann-pipnn/benches/kernels.rs	Adds benchmarks for partition top‑k, lower AAT, leaf top‑k, and full leaf workflow.
diskann-linalg/tests/sgemm_aat_lower.rs	New tests for lower-triangle AAT behavior and validation errors.
diskann-linalg/src/lib.rs	Adds public sgemm_aat_lower API with dimension checks.
diskann-linalg/src/faer.rs	Implements sgemm_aat_lower_impl using Faer triangular matmul.
Cargo.toml	Adds diskann-pipnn to workspace members and workspace dependencies.
Cargo.lock	Records the new diskann-pipnn package entry.
.github/workflows/ci.yml	Adds diskann-pipnn to CI test package lists.
.cargo/mutants.toml	Adds mutation-test exclusions for kernel code paths and equivalent transformations.

Comments suppressed due to low confidence (2)

diskann-pipnn/src/leaf_kernel.rs:651

• Same issue as the L2 arm: using max_simd for lower clamping can erase NaNs on the Scalar/Emulated backend, making NaN distances rankable. Clamp with lt_simd + select to preserve NaNs consistently.

        Metric::CosineNormalized => {
            let distance = F::splat(arch, 1.0) - dot;
            zero.max_simd(distance)
        }

diskann-pipnn/src/leaf_kernel.rs:664

• The cosine path also uses zero.max_simd(distance) for clamping, which can collapse NaNs to zero on the Scalar/Emulated backend (via f32::max). That contradicts the comment about preserving non-rankable NaNs and can change output ordering. Prefer an lt_simd + select clamp here as well.

            let distance = one - cosine;
            // Comparisons with NaN are false, so this explicit lower clamp
            // preserves non-rankable NaNs while matching the existing PiPNN
            // distance formulas for finite values.
            zero.max_simd(distance)

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