bench(cogs): steady-state CPU infer latency benches (ADR-163 T2)

Criterion benches over InferenceEngine::infer for cog-person-count and
cog-pose-estimation, on Device::Cpu with the real shipped safetensors
weights (asserts candle backend so the stub is never silently benched),
over a fixed CSI window after a warm-up forward.

HOST-MEASURED steady-state medians (idle box): ~305us each. This is the
recurring per-frame cost and is explicitly NOT the pose manifest's
cold_start_ms_avg=5.4 (a different measurement, weight-load included, taken
on ruvultra/RTX 5080) -- the two are labelled and not conflated.

Closes the ADR-159/160 deferred cog inference-latency item. No production-
code behavior change.

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/Cargo.lock

@@ -1015,6 +1015,7 @@ dependencies = [
  "candle-core 0.9.2",
  "candle-nn 0.9.2",
  "clap",
+ "criterion",
  "safetensors 0.4.5",
  "serde",
  "serde_json",
@@ -1034,6 +1035,7 @@ dependencies = [
  "candle-core 0.9.2",
  "candle-nn 0.9.2",
  "clap",
+ "criterion",
  "hex",
  "safetensors 0.4.5",
  "serde",