GpuExperienceCollectorConfig.use_noisy_nets and use_distributional were
both declared with "always enabled" comments on their default=true
setters. Zero conditional readers anywhere — the GPU kernel always
runs NoisyNet exploration and C51 distributional RL. Deleted fields +
defaults + training_loop setters. Kept noisy_sigma_init, num_atoms,
v_min, v_max since those are read.
enable_action_masking left for human review — has real gating chain
across collector config + trainer struct + training loop.
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