feat(benchmark): per-node cost and bottleneck attribution for a run

--benchmark <path> reports cumulative CPU and wall time per node and
names the node pacing the run. The pacing node is located from sampled
channel occupancy, not from time-in-node: backpressure inflates
time-in-node for everything downstream of the real bottleneck, so the
obvious measure names the victim rather than the cause.

Sampling starts with the network and stops before it is destroyed.
Channel fill is instantaneous and everything has drained by shutdown, so
a single read at the end reports an idle pipeline however congested it
was.

kill -USR1 dumps the table from a running or wedged process. Channel
occupancy identifies a stalled node -- full input, empty output --
without a debug build or a debugger, which is the difference between
diagnosing the AR-004 hang in seconds and reproducing it under gdb.

Two knobs this exposes for measurement rather than sets: SAE_CV_THREADS,
because OpenCV's TBB arena and KPN's thread-per-node are two schedulers
unaware of each other on the same cores; and SAE_CUDA_BLOCKING_SYNC,
because the default spin-wait held the embedder thread at 99.7% user
time while nvidia-powerd cut the GPU's clock from 1005 to 210 MHz.
Neither default changes until a measurement says it should.

TRACES: VR-015 | PR-004

tests/CMakeLists.txt

@@ -27,6 +27,7 @@ add_executable(sae_tests
     test_replay_fixtures.cpp
     test_embedding_dump.cpp
     test_audio_signature.cpp
+    test_benchmark.cpp
     ${CMAKE_SOURCE_DIR}/src/backends/gemm_backend.cpp
     ${CMAKE_SOURCE_DIR}/src/gallery/gallery_store.cpp
     ${CMAKE_SOURCE_DIR}/src/audio_signature.cpp
@@ -70,6 +71,10 @@ target_compile_definitions(sae_tests PRIVATE
 target_link_libraries(sae_tests PRIVATE
     Catch2::Catch2WithMain
     nlohmann_json::nlohmann_json
+    # VR-015: test_benchmark.cpp includes src/benchmark.hpp, which reads KPN's
+    # diagnostics structs. Header-only — no KPN network is constructed here, so
+    # the cost attribution stays testable on CI's GPU-free N100.
+    kpn
     ffmpeg_libs
     ${OpenCV_LIBS}
     ${HDF5_CXX_LIBRARIES})