seg_tree_update and seg_tree_insert used non-atomic
tree[node] = tree[2*node] + tree[2*node+1] with 8192 threads racing
on shared internal nodes. On H100 (132 SMs), all threads execute
simultaneously causing data races and hangs.
Replace with atomicAdd delta propagation: each thread computes
delta = new_leaf - old_leaf, then atomicAdd to every ancestor.
Commutative + associative = race-free. O(log n) per thread,
hardware-accelerated on SM 9.0.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
ae65740a3d