BP+penalty control result: mode 2 (intrinsic credit quality) confirmed REAL

BP + lam=1e-2 ||f||^2 penalty trained for 30 epochs (s42):
  ep 30 final: test_acc 0.5303
  margin vs DFA-shallow 0.349: +18.13 pp

The 2x2 accuracy grid:
                  no penalty       with penalty
  BP             0.609            0.530
  DFA            0.308            0.363

Penalty effect on BP: -8 pp (capacity regularization cost)
Penalty effect on DFA: +5.5 pp (rescue from active harm)

Mode 2 (intrinsic credit quality) is confirmed REAL by this control:
even after the penalty's capacity cost, BP achieves +18 pp depth
utilization. DFA under the same penalty achieves only +1.4 pp. The
difference (~17 pp) cannot be attributed to capacity loss — it is
genuine credit-quality cost of random feedback vs true backprop
gradient.

This validates the round 19 'two distinct failure modes' framing:
mode 2 is not a penalty-induced regularization artifact.

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