Adds --per-horizon-logit-sampling-stride CLI flag to fxt-backtest run/sweep
+ harness instrumentation that DtoHs alpha_probs every N events and
accumulates per-horizon mean/stddev/min/max + 10-bin histogram.
Emitted at end of CRT.diag block as:
per_horizon_logit_diag h{N}: count=... mean=... std=... min=... max=... hist=[...]
Goal: distinguish whether Smoke 2's uniform mean_run_len 1.04x across
horizons is caused by uniform per-horizon LOGITS (-> Task 11 scope
incomplete; heads_w1/w2/gate/main need block-diagonal too) or by
DIFFERENTIATED logits with collapsed downstream (-> different fix).
Default stride=None (disabled, zero overhead). With stride=1000 on
2M events, ~2000 DtoD stops x ~50us = ~100ms total overhead per run.
Per feedback_no_htod_htoh_only_mapped_pinned: uses mapped-pinned buffer
(single 5xf32 allocation reused on every sampling tick).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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