test: add Catch2 unit test suite (gallery, calibration, tracking, similarity)

GPU-free, model-free tests for the pure logic: gallery HDF5 save/load
round-trips (actors, embeddings, embedded calibration) and legacy JSON
read back-compat; the calibration sigmoid fit, boundary inversion, and the
in-memory hash-keyed cache reuse/staleness; TrackGallery's diversity-buffer
eviction, novelty/spread safety gates, and promotion; FaceTracker's IoU/
embedding association and cross-cut track revival; and the GEMM similarity
backend (forced to CPU so the suite runs without a GPU).

Verified: all 39 test cases / 1640 assertions pass (cmake -DSAE_BUILD_TESTS=ON).

tests/CMakeLists.txt

@@ -0,0 +1,45 @@
+# ── Unit tests ────────────────────────────────────────────────────────────────
+# Pure, GPU-free, model-free tests. The similarity tests compile the GEMM backend
+# directly with SAE_GEMM_CPU so the suite builds and runs on a machine without a
+# GPU regardless of the main build's SAE_GEMM_BACKEND selection.
+
+find_package(Catch2 3 QUIET)
+if(NOT Catch2_FOUND)
+    include(FetchContent)
+    FetchContent_Declare(
+        Catch2
+        GIT_REPOSITORY https://github.com/catchorg/Catch2.git
+        GIT_TAG        v3.5.3
+    )
+    FetchContent_MakeAvailable(Catch2)
+endif()
+
+add_executable(sae_tests
+    test_similarity.cpp
+    test_calibration.cpp
+    test_gallery_store.cpp
+    test_face_utils.cpp
+    test_track_gallery.cpp
+    test_face_tracker.cpp
+    ${CMAKE_SOURCE_DIR}/src/backends/gemm_backend.cpp
+    ${CMAKE_SOURCE_DIR}/src/gallery/gallery_store.cpp
+)
+target_include_directories(sae_tests PRIVATE ${CMAKE_SOURCE_DIR}/src)
+# SAE_GEMM_CPU: build the CPU reference GEMM regardless of the main backend.
+# SAE_MODELS_DIR: config.hpp (pulled in by track_gallery.hpp) bakes model paths.
+target_compile_definitions(sae_tests PRIVATE
+    SAE_GEMM_CPU
+    SAE_MODELS_DIR="${SAE_MODELS_DIR}")
+# gallery_store.cpp + gallery_calibration.hpp use nlohmann/json and HDF5
+# (galleries are HDF5-native, see src/gallery/gallery_store.cpp); face_utils.hpp
+# and the calibration GEMM pull in OpenCV (calib3d/imgproc/core) via types.hpp.
+target_link_libraries(sae_tests PRIVATE
+    Catch2::Catch2WithMain
+    nlohmann_json::nlohmann_json
+    ${OpenCV_LIBS}
+    ${HDF5_CXX_LIBRARIES})
+target_include_directories(sae_tests PRIVATE ${HDF5_INCLUDE_DIRS})
+
+include(CTest)
+include(Catch)
+catch_discover_tests(sae_tests)