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/nodes/face_aligner_node.hpp

@@ -24,11 +24,15 @@ struct FaceAlignerFunc {
         crops.reserve(sf.faces.size());
 
         for (auto& face : sf.faces) {
-            cv::Mat crop = align_face(sf.source.image, face.landmarks);
+            // The AR-030 misfit comes from the transform the warp already needs,
+            // so visibility costs no extra fit.
+            float residual = -1.f;
+            cv::Mat crop = align_face(sf.source.image, face.landmarks, &residual);
             if (crop.empty()) {
                 std::cerr << "[face_aligner] degenerate detection skipped\n";
                 continue;
             }
+            face.alignment_residual = residual;
             good_faces.push_back(face);
             crops.push_back(std::move(crop));
         }