Round 35: SB and CB also show data-agnostic Mode 1 growth on random targets

- experiments/cifar_resmlp.py: add --methods filter and --random_targets flag;
  extend compute_diagnostics to log hidden_norms_per_layer and bp_grad_norms_per_layer
- paper/main.tex §3 ¶1: broaden random-target finding to all 3 fixed-feedback methods
  (DFA: ||h_L||=14510, SB: ||h_L||=6225, CB: ||h_L||=19974 at ep 3, all at chance acc)
- paper/main.tex Appendix J: extended with cross-method smoke-test table

This generalizes the §3 mechanism story from 'DFA-specific' to 'all 3 audited
fixed-feedback local-credit methods'. Combined with rounds 32-34, the proximate
cause of Mode 1 (a) is now well-localized:
  - Not requires residual skip (round 33 H2 walkback)
  - Not requires task signal (round 34 random targets, DFA)
  - Not DFA-specific (round 35 random targets, SB+CB)

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

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