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/types.hpp

@@ -63,6 +63,13 @@ struct DetectedFace {
     cv::Rect2f                   bbox;
     std::array<cv::Point2f, 5>   landmarks;
     float                        confidence{0.f};
+
+    // AR-030 visibility: RMS landmark misfit, in canonical 112×112 pixels, left
+    // over after the best similarity fit to the ArcFace template. Rises with
+    // out-of-plane pose and with occlusion; blind to in-plane roll and to face
+    // size, both of which the fit absorbs. Set by the aligner, which is where
+    // the transform is computed; -1 until then.
+    float                        alignment_residual{-1.f};
 };
 
 // ── Pipeline messages ─────────────────────────────────────────────────────────