Move tau sampling and factored noise generation from host-side ChaCha8
RNG + mapped-pinned upload to device-side xorshift32 kernels. This
eliminates all host-side RNG from the step pipeline, unblocking CUDA
Graph capture for Graphs A and C.
New kernels:
- rl_sample_tau: per-batch xorshift32 generates tau [B, N_TAU] ~ U(0,1)
- rl_sample_noise: factored noise f(x)=sign(x)√|x| for NoisyLinear
Both kernels self-seed from alloc_zeros on first call (zero memcpy).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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