Add BP support sparsity analysis: threshold sweep + gradient histograms

A1 Synthetic: all methods have >93% support at τ=1e-6 (gradients rarely zero)
A2 CIFAR: massive gap — BP 98.4% vs DFA 0.4% vs SB 21% vs CB 3%
  DFA-trained CIFAR networks have near-zero BP gradients for 99.6% of samples
  This explains why Gamma is unreliable for CIFAR non-BP methods

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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