Adds 12-entry u64 ptr table + 12-entry i32 len table on GpuDqnTrainer
for the fused NaN-check kernel. Allocated as zeros; populated once
in next commit (slot pointers stable across trainer lifetime).
Slot 31 (deferred ensemble) entry will be 0 — kernel skips that block.
Two separate buffers chosen over packed-stride layout for direct
ABI match with the kernel signature (const float* const* buf_ptrs,
const int* buf_lens). Avoids stride-alignment concerns.
Unused yet — populate + wrapper + call-site replacement in next commits.
82b6bd369e