dqn_distributional_q_bwd: restructure from 21 threads/block (Q_N_ATOMS)
to 256 threads/block. Each thread maps to one (action, atom) pair.
Softmax reduction stays in warp 0; all 231 threads write gradients in
parallel. Block tree-reduce for loss accumulation (no intra-block
atomicAdd). Was 23% of L40S GPU time at 167μs/call.
Convert ALL remaining 46 launch_builder calls to raw_launch across
dqn.rs (9), ppo.rs (6), outcome_head.rs (7), frd.rs (4), noisy.rs (3),
bucket_routing.rs (5), cfc/step.rs, aux_trunk.rs, snap_features.rs,
loss.rs. Zero launch_builder in crate (1 comment only).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
a5b39ecc61