Mirrors the alpha_train CLI flag (--kernel-step-trace <path>) into
fxt-backtest. The PerceptionTrainerConfig already accepts the path;
this commit just exposes the CLI surface and propagates through:
fxt-backtest run --kernel-step-trace <path>
-> BacktestHarnessConfig.kernel_step_trace_path
-> PerceptionTrainerConfig.kernel_step_trace_path
Sweep YAML gains a `kernel_step_trace: Option<PathBuf>` base field.
Argo lob-backtest-sweep-template gains a `kernel-step-trace` workflow
parameter (default disabled; when set, --features kernel-step-trace
is passed to cargo build AND --kernel-step-trace to fxt-backtest).
Gated behind the `kernel-step-trace` Cargo feature (matching alpha_train).
When disabled (default), zero overhead.
Enables per-step JSONL diagnostic emission from inference kernels --
needed for Smoke 2 deep-dive after sampling histograms (in parallel
investigation) localize the per-horizon dynamics bottleneck.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
ff9207e7bb