Previous L40S 15-epoch repro fired the wired NaN diagnostic with
flagged=[2=on_b_logits, 3=mse_loss_scalar, 6=grad_buf, 7=save_current_lp,
8=save_projected] while value stream (flag 1) and pre-forward params
(flags 4-5) stayed clean. That isolates the corruption to the branch
advantage GEMM/activation but leaves one open question: is the GEMM
input (h_s2, shared between value and branch heads) finite or already
NaN?
Flag 12 = save_h_s2 covers the post-trunk activation. Outcomes:
- flag 12 clean + flag 2 NaN → branch GEMM overflowed (advantage-side
fp32 overflow under post-S&P + adversarial reward stress).
- flag 12 NaN → trunk encoder corrupted upstream of both heads.
Mechanically: one new check_nan_f32 call against save_h_s2 in
run_nan_checks_post_forward, names array slot 12 updated rsv12 →
save_h_s2 in training_loop.rs error message. Same ~5µs cost as the
other 12 checks. Slots 13-15 still reserved.
Per `feedback_no_partial_refactor.md` (extending the NaN diagnostic
contract atomically) and `feedback_no_stubs.md` (rsv12 was a real
reservation; now consumed).
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