Existing nan_flags_buf [16] covers 13 buffers with per-step NaN checks
inside the captured training graph. But read_nan_flags() was only
called when training guard's halt_nan fired — which checks pinned-
readback grad_norm. NaN-clamped-to-zero gradients reach the pinned
scalar as 0, not NaN, so halt_nan never fires for the explosion case.
The collapse path (halt_grad_collapse, grad < 0.01) was firing instead
without reading the flags. Add flag readback in that path when
gr.raw_grad_norm < 1e-6 (suspicious zero) — logs NaN-CLAMPED-TO-ZERO
with flagged buffer names. Genuine near-zero gradients get a separate
"no NaN flags set" log so we can distinguish the two cases.
This should pinpoint which kernel produces the F1 ep2 NaN first
(C51 KL projection? IQN aux? CQL? aux heads?). Diagnostic only —
keep after fix lands; correct gate for future regressions.
d1808df14c