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

src/config.hpp

@@ -32,6 +32,14 @@ struct Config {
     // scripts/optimizer/SCHEMA.md) for offline threshold-sweep replay via sae_kpn.
     std::string dump_embeddings_path;
 
+    /// TRACES: VR-015 | PR-004
+    // When set, write a per-node timing and bottleneck report here (src/
+    // benchmark.hpp) and print it at shutdown. Costs one background thread
+    // reading relaxed atomics on a timer, so it is safe to leave on, but a
+    // measurement run should still be isolated (nothing else on the GPU).
+    std::string benchmark_path;
+    int         benchmark_interval_ms{100};   // channel-occupancy sampling period
+
     // ── Sampling ─────────────────────────────────────────────────────────────
     float  sample_fps{1.0f};     // frames to analyse per second of movie
     float  max_decode_fps{0.f};  // wall-clock cap on source decode rate (0 = uncapped)