Add reproducers for pitfalls 1-3 in CHECKLIST.md

Each bug from the catalog has a synthetic reproducer that runs in <1 sec
without GPU:

  Bug 1: x.norm(-1) on a 2x2 tensor returns 1.143 (L_{-1} of whole tensor)
         instead of [5, 10] (per-row L_2 along dim=-1).
  Bug 2: F.cosine_similarity(a, b) with ||b||=5e-10 returns +0.000905
         instead of the true +0.018101. The clamp (eps=1e-8) underestimates
         the divisor 20x.
  Bug 3: 5e-10 in fp16 -> 0 (underflows smallest subnormal ~6e-8).
         Downstream F.cosine_similarity returns NaN. bf16 works because it
         shares fp32's exponent range.

Bugs 4-6 (Bs reproducibility, aggregation, layer-0 dominance) require a
trained network and are demonstrated inside audit_table and
ablation_decision_utility.

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