Replaces the ad-hoc scattered fold-boundary reset calls with a single
registry-driven iteration. Adding new fold-reset state now requires
adding a registry entry AND a dispatch arm in reset_named_state in the
same commit (Invariant 2 Wire-It-Up).
Step 3.4 correction: plan_state entry removed from the registry —
plan_state_buf exists only in GpuBacktestEvaluator (val path), not in
the training-path fused ctx. No training-side fold reset is applicable.
New behaviour: isv_learning_health, isv_sharpe_ema, isv_q_means are
now properly reset to baseline at each fold boundary (previously unset,
which allowed signals from fold N to bias fold N+1 initialisation).
Tests: 3 registry unit tests pass; cargo check -p ml clean (8 pre-existing
warnings only, no new).
Authority: spec §4.A.1. Plan 1 Task 3.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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