Adds envelope-detector floor on popart_sigma so single-account tail events
within a batch are captured instead of diluted by the batch-mean variance.
Kernel changes (rl_popart_normalize.cu):
- warp_reduce_max helper alongside existing warp_reduce_sum
- Pass 1 extension: per-thread local_max_abs via fmaxf(fabsf(r))
- 3rd shared-mem bank for max_abs reduction (s_max_abs[])
- envelope-detector inside tid==0 block: fast-up, slow-down (α_d=0.01)
- new_sigma = fmaxf(new_sigma, max_r_ema) before write
- CRITICAL (Issue β): Pass 3 broadcast slots moved block_dim*2 → block_dim*3
Trainer launch update: smem_bytes = (block_x * 3 + 3) * sizeof(f32).
Per Theorem 6 (spec v3): at Run #8 step 7377 with r_tail=-19.45, the
envelope captures 19.45 instantly via fast-up path; sigma jumps from
2.327 → 19.45 in one step. The popart_v_correct kernel handles the
σ discontinuity affinely so V regression doesn't destabilize.
Decay phase: α_d=0.01 (69-step half-life); max_r_ema returns to typical
batch-max baseline over ~200 steps after a single shock.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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