Replace host-side `current_t` scalar parameter in experience_env_step
with a GPU-resident counter buffer. The kernel now reads the timestep
index from device memory (step_counter_gpu[0]) instead of receiving it
as a kernel argument that changes every iteration. A tiny single-thread
step_counter_advance kernel increments the counter after each env_step.
This eliminates per-timestep host→GPU parameter variation in the 100-
iteration experience collection loop, making all iterations dispatch
identical kernel argument sets — a prerequisite for future CUDA Graph
capture of the timestep sequence.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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