perf: back the CPU similarity GEMM with OpenBLAS

The CPU path was a scalar triple loop. It is the correctness oracle for the GPU
backends, but it is also what CI runs — there is no GPU on the N100 host — and
since AR-003 removed the per-frame face cap, a crowded frame now scores many
faces against a library-scale gallery. Scoring one face against 5000 embeddings
is 2.6 MFLOP; in scalar that does not hold up (AR-027).

S(g,f) viewed as row-major [n_faces x n_gallery] is exactly query * gallery^T,
so the loop nest collapses into a single cblas_sgemm.

OpenBLAS is optional in the build: found via pkg-config, and the scalar path
remains when it is absent so no hard dependency is added and the two can be
diffed when a similarity looks wrong. The configure step warns rather than
failing, since a developer without it should still get a working tree.

The test target links it too. Without that the suite compiles the scalar
fallback while the builder image ships CBLAS, so CI would be verifying a kernel
that is not the one running in production — the same class of mistake as testing
a path the gate never executes.

Recorded as required (not optional) in the DP-007 image, for the same reason.

Suite: 92 cases, 6136 assertions, with CBLAS compiled in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-026, AR-027, DP-007 | SR-001

src/backends/gemm_backend.cpp

@@ -34,9 +34,23 @@ constexpr int kDim = 512;
 
 #if defined(SAE_GEMM_CPU)
 
+#if defined(SAE_GEMM_CBLAS)
+#include <cblas.h>
+#endif
+
 // ── CPU reference engine ──────────────────────────────────────────────────────
-// Portable, dependency-free path used for CI and as the correctness oracle for
-// the GPU backends. The gallery is L2-normalised (as are the queries), so each
+// Used for CI and as the correctness oracle for the GPU backends.
+//
+// TRACES: AR-026, AR-027 | SR-001
+// Backed by CBLAS (OpenBLAS) when available, falling back to a scalar loop when
+// not. The fallback is portable but scales badly: scoring one face against a
+// 5000-embedding gallery is 2.6 MFLOP, and a crowded frame multiplies that by
+// the face count. Since AR-003 removed the per-frame face cap and CI has no GPU,
+// the CPU path is now the one that has to hold up under a library-scale gallery
+// (AR-027) rather than merely be correct.
+//
+// The fallback is kept rather than made mandatory so the build has no hard new
+// dependency, and so the two can be diffed when a similarity looks wrong. The gallery is L2-normalised (as are the queries), so each
 // similarity is a plain dot product. S is stored column-major to match the GPU
 // backends: the gallery similarities for face fi start at result + fi*n_gallery.
 class SimilarityEngine final : public ISimilarityEngine {
@@ -47,7 +61,13 @@ public:
                    gallery_row_major + static_cast<size_t>(n_gallery) * kDim)
     {
         host_sims_.resize(static_cast<size_t>(max_faces_) * n_gallery_);
-        std::cerr << "[similarity] CPU reference engine: gallery resident in host RAM ("
+        std::cerr << "[similarity] CPU engine ("
+#if defined(SAE_GEMM_CBLAS)
+                  << "CBLAS"
+#else
+                  << "scalar fallback — no CBLAS; expect poor scaling on a large gallery"
+#endif
+                  << "): gallery resident in host RAM ("
                   << (gallery_.size() * sizeof(float)) / (1024 * 1024) << " MiB)\n";
     }
 
@@ -58,7 +78,18 @@ public:
         if (n_faces > max_faces_)
             throw std::runtime_error("SimilarityEngine: n_faces exceeds max_faces");
 
-        // S(g, f) col-major = dot(gallery[g], query[f]).
+        // S(g, f) col-major = dot(gallery[g], query[f]). Viewed as row-major
+        // [n_faces x n_gallery] that is exactly query * gallery^T, so it is one
+        // GEMM rather than a loop nest.
+#if defined(SAE_GEMM_CBLAS)
+        cblas_sgemm(CblasRowMajor, CblasNoTrans, CblasTrans,
+                    /*M=*/n_faces, /*N=*/n_gallery_, /*K=*/kDim,
+                    /*alpha=*/1.0f,
+                    query_row_major, /*lda=*/kDim,
+                    gallery_.data(), /*ldb=*/kDim,
+                    /*beta=*/0.0f,
+                    host_sims_.data(), /*ldc=*/n_gallery_);
+#else
         for (int f = 0; f < n_faces; ++f) {
             const float* q = query_row_major + static_cast<size_t>(f) * kDim;
             float* out = host_sims_.data() + static_cast<size_t>(f) * n_gallery_;
@@ -69,6 +100,7 @@ public:
                 out[g] = acc;
             }
         }
+#endif
         return host_sims_.data();
     }