Baguette's 512 MiB logit gradient fits the gradient bank's blocks 0..37 slice

Arena reduction for the RTX 3070 (Alpha commit 065122bd): the bfloat16 logit gradient (4096 x 65536 x 2 bytes = 2^29) is placed inside the binary32 gradient bank's slice for decoder blocks 0..37, which the backward (blocks 79 down to 0) writes only after the logit gradient's last reader. Geometry: vocabulary 65536, width 576, QKV width 960, feed 1536; a block holds 576960 + 576576 + 35761536 + 2576 = 3,540,096 binary32 gradients after the 65536576 embedding gradients. The statement checks the slice start is 2 MiB aligned, that 38 blocks hold the plane, and that 37 do not (so 38 is the least slice).